Update libexecinfo man page for FreeBSD

Sponsored by:	DARPA, AFRL
Use tvtohz() to convert a socket buffer timeout to a tick value rather
than using a home-rolled version.  The home-rolled version could result
in shorter-than-requested sleeps.

Reported by:	Vitja Makarov <vitja.makarov@gmail.com>
MFC after:	2 weeks
libc: Make various internal file descriptors from fopen() close-on-exec.
Introduce callout_init_rm() so that callouts can be used in conjunction
with rmlocks. This works only with non-sleepable rm because handlers run
in SWI context. While here, document the new KPI in the timeout(9)
manpage.

Requested by:	adrian, scottl
Reviewed by:	mav, remko(manpage)
libc: Access some unexported variables more efficiently (related to stdio).