Add clock_nanosleep()

Add a clock_nanosleep() syscall, as specified by POSIX.
Make nanosleep() a wrapper around it.

Attach the clock_nanosleep test from NetBSD. Adjust it for the
FreeBSD behavior of updating rmtp only when interrupted by a signal.
I believe this to be POSIX-compliant, since POSIX mentions the rmtp
parameter only in the paragraph about EINTR. This is also what
Linux does. (NetBSD updates rmtp unconditionally.)

Copy the whole nanosleep.2 man page from NetBSD because it is complete
and closely resembles the POSIX description. Edit, polish, and reword it
a bit, being sure to keep any relevant text from the FreeBSD page.

Reviewed by: kib, ngie, jilles
MFC after: 3 weeks
Relnotes: yes
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D10020