fix few more typos in comments, messages and documentation.

ndp: Show U for Unreachable

ndp: Sync usage and SYNPOSIS with reality

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).

Sort some lists. Use EXIT STATUS instead of RETURN VALUES.

Use .Cm instead of .Li, same as arp.8.

Add IPV6CTL_AUTO_LINKLOCAL and ND6_IFF_AUTO_LINKLOCAL toggles which
control the automatic creation of IPv6 link-local addresses when an
interface is brought up.

Taken from FreeBSD.

branches: 1.26.2;
If IPv6 is disabled for an interface, mark all addresses as tentative.
If enabled, check for a duplicated link-local address and abort enabling
as per RFC 4862, section 5.4.5. If allowed to enable, perform DAD
on the tentative addresses.

Taken from FreeBSD.

branches: 1.25.6; 1.25.12;
Update date.

Teach ndp(8) about override_rtadv.

Simplify markup by not using .Xo/.Xc.

branches: 1.22.28;
NDP-related improvements:
RFC4191
- supports host-side router-preference

RFC3542
- if DAD fails on a interface, disables IPv6 operation on the
interface
- don't advertise MLD report before DAD finishes

Others
- fixes integer overflow for valid and preferred lifetimes
- improves timer granularity for MLD, using callout-timer.
- reflects rtadvd's IPv6 host variable information into kernel
(router only)
- adds a sysctl option to enable/disable pMTUd for multicast
packets
- performs NUD on PPP/GRE interface by default
- Redirect works regardless of ip6_accept_rtadv
- removes RFC1885-related code

From the KAME project via SUZUKI Shinsuke.
Reviewed by core.

change description of -I to meet the current implementation. bump date

with -i, flag on command line and flag displayed are different, and it does
nothing but confuse users. sync them.

for -nud and such, you need "-- -nud" due to posix arg parsing.

sync w/kame

comment out paragraph that does not fit to NetBSD

recover backward compatibility in -I behavior. sync w/kame

more KNF/nroff. from deraadt via kame

more posix-compliant arg parsing. sync with kame. help from deraadt

Drop a space, and two grammar fixes.

use new SIOCGIFINFO_IN6. random other cleanups.

Generate <>& symbolically. I'm avoiding .../dist/... directories for now.

Punctuation nits.

Xref tcpdump 8, not 1.

sync with latest kame tree. ndp -n -a is prettier with longer addresses.
-l is obsolete (igored).

For commands and utilities, use EXIT STATUS rather than RETURN VALUES or
DIAGNOSTICS as appropriate (and documented in mdoc(7)).

sync with more recent kame code.
accept scoped address notation.

branches: 1.7.4;
perform neighbor unreachability detection on p2p links (spec requires
it for bidir p2p links).
improve -i in ndp(8) to allow tweaking per-interface ND flag on.
fix ndp(8) infinite loop on certain routing table setup.

Typo.

remove net.inet6.ip6.nd6_proxyall sysctl.
support "ndp -s <ip6> <mac> proxy" for proxy NDP.

add -I and -l. cleanup on screen formatting.
(sync with latest KAME)

branches: 1.3.4;
fix routing socket alignment issue on alpha.
sync with more recent KAME code.

PR: 8305

add NetBSD RCS ID.

ndp, "arp"-alike command for IPv6.