rc.subr: ensure run_rc_script return code is not masked by DebugOff

Ensure that run_rc_script returns the true return code of the
rc command and not the return code of DebugOff

Reviewed by: sjg
Approved by: kp
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46626