Don't use set_rcvar as it has been removed from rc.subr

This patch fix a (minor) bug in the FreeBSD rc.d rmilter script. Before the patch:
```
# service rmilter.sh restart
/usr/local/etc/rc.d/rmilter.sh: set_rcvar: not found
# pkg info | grep rmilter
rmilter-1.6.1_1                Milter that performs spamd, clamav, and spf checks
# freebsd-version 
10.0-RELEASE-p9
```