Remove in-kernel handling of Router Advertisements

This is much better handled by a user-land tool.
Proposed on tech-net here:
https://mail-index.netbsd.org/tech-net/2020/04/22/msg007766.html

Note that the ioctl SIOCGIFINFO_IN6 no longer sets flags. That now
needs to be done using the pre-existing SIOCSIFINFO_FLAGS ioctl.

Compat is fully provided where it makes sense, but trying to turn on
RA handling will obviously throw an error as it no longer exists.

Note that if you use IPv6 temporary addresses, this now needs to be
turned on in dhcpcd.conf(5) rather than in sysctl.conf(5).

tests: add tests for the validateion of net.inet6.ip6.temppltime

branches: 1.32.4; 1.32.6;
Make this test somewhat deterministic - far fewer races, and most
of what are left are "race for the bus" type - if we lose, we just
wait for the next one ... slower but still reliable.

There are two exceptions ... when starting more than one rtadvd
(on different routers) we expect to receive an RA from each, but
all that we can check is that we received the (at least) right number
of RAs. It is possible (though unlikely) that one router sent two
before another sent any, in which case we will not have the data we
expect, and a sub-test will fail.

Second, there is no way to know for sure that we have waited long
enough when we're waiting for data to expire - in systems with
correctly working clocks that actually measure time, this should not
be an issue, if data is due to expire in < 5 seconds, and we wait
5 seconds, and the data is still there, then that indicates a
failure, which should be detected. Unfortunately with QEMU testing
time just isn't that reliable. But fortunately, it is generally the
sleep which takes longer, while other timers run correctly, which is
the way that makes us happy...

While here lots of cleanups - everything from white space and
line wrapping, to removing superfluous quotes and adding some
(but probably not enough) that are not (though given the data is
all known here, lack of quotes will rarely hurt.)

Also take note of the fact that current rtadvd *cannot* delete its
pidfile, so waiting for that file to be removed is doomed to failure.
Do things in a way that works, rather than simply resorting to assassination.

Because we do a lot less "sleep and hope it is long enough" and more
"wait until it is observed to happen" the tests generally run in less
elapsed time than before (20% less has been observed.) But because we
"wait until it is observed to happen" rather than just "sleep and hope
it is long enough" sometimes things take longer (and when that happens,
we no longer fail). Up to 7% slower (overall) has been observed.
(Observations on an amd64 DomU, no idea yet as to what QEMU might observe.)

Let rtadvd not use syslog for logging

Thanks to christos@ now rtadvd can log via stderr instead of syslog
by -D option.

Address PR bin/52701

Kill rtadvd surely even if the tests fail in the middle

It may help PR bin/52701.

Purge all related L2 caches on removing a route

The change addresses situations similar to PR 51179.

Don't create a permanent L2 cache entry on adding an address to an interface

It was created to copy FreeBSD, however actually the cache isn't
necessary. Remove it to simplify the code and reduce the cost to
maintain it (e.g., keep a consistency with a corresponding local
route).

branches: 1.27.2;
Change the default value of DEBUG of stable tests to false

Provide a more robust regexp for time formats of 1day-ish

Add tests for expiration of default router and prefix entries

branches: 1.24.2;
Remove a check added wrongly

Add tests for net.inet6.ip6.prefer_tempaddr

Remove extra checks and cleanup

Cope with tentative state

Add a test case for IPv6 temporary address

Check autoconf flag

Fix typo

Restore multiple_routers_single_prefix_cleanup removed wrongly

Reduce unnecessary wait

Add a test case for exceeding the number of maximum prefixes

The test case pinpoints purge_detached.

Add tests for multiple routers with a single prefix

Fix the description of a test

Add tests for multiple routers

Unify common routines

Avoid using /var/run/rump.rtadvd.pid

Add a test case that deletes auto-configured addresses

Improve stability of the tests

- Do ifconfig -w 10 after ifconfig up
- Accept /1d0h0m..s/ in addition to /23h59m..s/ for expiration time
- Prevent new RA messages from coming after flushing entries

The changes should fix flapping of test results on babylon5.

Add tests for flushing prefix and default router entries

Share rump_server start/stop and interface creation/destruction functions

The common functions store socks of rump_servers, interfaces of rump_servers
and buses that intefaces connect and allow to destroy them with common
functions without specifying which socks, interfaces and buses we should
destroy.

This change reduces lots of similar setup/cleanup codes.

Accept DEBUG environment variable

By doing so, we can easily turn DEBUG on/off without modifying
the ATF scripts.

Make test names self-descriptive

+ -lrumpdev

branches: 1.2.2;
Fix up the header

Remove unnecessary shebang and add missing keyword expansion,
copyright and license.

Add tests for RA

From s-yamaguchi@IIJ (with some tweaks by me)