Tweak markup in _lwp_*(2) manual pages

Use .Rv where possible. Some manual pages claimed that the error "is
returned", but RTFS and some quick testing indicate that this is wrong.

The commit message from 2003 says that:

Note our current implementation mis-matches [man pages] slightly
(error codes are stuffed into errno, where they should simply be
returned by these calls). This will be addressed shortly.

That hasn't happened in the 20 years, so we might as well make the man
pages reflect the reality.

Fix a couple of typos

branches: 1.11.4;
Remove trailing whitespace.

make _lwp_park return the remaining time to sleep in the "ts" argument
if it is a relative timestamp, as discussed in tech-kern.
XXX: pullup-8

Fix prototype in SYNOPSIS, mark up NULL with Dv.

expose __lwp_park60

branches: 1.7.4; 1.7.8; 1.7.28;
Convert TNF licenses to new 2 clause variant

branches: 1.6.6;
remove extraneous "has"

branches: 1.5.2;
Change the signature of _lwp_park() to accept an lwpid_t and second
hint pointer, but do so in a way that remains compatible with older
pthread libraries. This can be used to wake another thread before the
calling thread goes asleep, saving at least one syscall + involuntary
context switch. This turns out to be a fairly large win on the condvar
benchmarks that I have tried.

- Sync with reality.
- Fix date.

Sort ERRORS section.

Drop trailing whitespace.

Manual pages for new LWP system calls. Not enabled in the Makefile.