tighten up /etc/rc.conf regex

I've run into a fun issue on FreeBSD 10 when trying to disable `syslogd`,
and enable `rsyslogd`.

Initially, the /etc/rc.conf config file may look something like this

    syslogd_enable="YES"
    rsyslogd_enable="NO"

And after chef it *should* look this

    rsyslogd_enable="YES"
    syslogd_enable="NO"

However, because the regex isn't as tight as it could be, it will search for
`syslogd_enable="(\w+)"`, and erroneously match the `rsyslogd` line, which leads
Chef to believe that `syslogd` is enabled, when in reality it isn't.  This change
simply anchors the regex with `^` so the pattern must match at the beginning of
the line.

PS: This is my first time submitting a change to Chef after the move off of Jira, but I've already signed the CLA and am number 753 on the approved contributors list.
In these cases I think what we're testing is that we set the value appropriate for the correct line in the rc.d file, so it would be more appropriate to say something like it "sets enabled based on the exact match (false)".