Backport patch 872594b from upstream

This is a FreeBSD specific fix to the handling of rcvars -- this fixes
the case where the rcvar did not previously exist in /etc/rc.conf or
other places the rc system might look for them.

https://github.com/ansible/ansible-modules-core/commit/872594b49a69a1f3795e0de3f7cf0194b6bdfd53

PR: 214322
Reported by: Thomas Steen Rasmussen
Approved by: lifanov@mail.lifanov.com (maintainer)