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.
Oh yeah we do that deliberately for why-run based `$REASONS`.

I think I'd still set it to false in both place, because we may want to rip this hack out at some point, and it'll do the person ripping this code out a favor if we're explicitly setting it correctly before.

Sorta YMMV, but I'd leave it like this.