rpcbind: Do not use signal-unsafe functions in SIGTERM handler

syslog(3), routines used in write_warmstart(), and exit(3) are all
signal-unsafe. Instead, set a signal-safe flag and check the flag in the
rpcbind main loop to shutdown safely.

PR: 224503
Reviewed by: kib, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D13728