sysrc(8): Exit with failure on API error

Fix exit status when f_sysrc_set() fails. Errors in the underlying API
provided by bsdconfig(8) -- /usr/share/bsdconfig/sysrc.subr -- were not
being communicated back to the command-line. This was affecting ansible
modules using sysrc as they were not able to accurately test for error.

PR: bin/211448
Reported by: Christian Schwarz <me@cschwarz.com>
MFC after: 3 days
X-MFC-to: stable/11
Sponsored by: Smule, Inc.