libsys: don't expose sigwait wrapper

Long ago (e129c18a83ef) sys_sigwait was wrapped to prevent sigwait()
from returning with EINTR. Through a series of changes this wrapper
become libc_sigwait which was internal to libc and used solely in the
interposing table. To support a move of sigwait back to libc, move this
wrapper into libsys and rename it with an __libsys_ prefix.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44238