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.
Also, `sh(1)` syntax doesn't allow for spaces around `=` for variable declaration. ie.

``` sh
foo=bar
foo= bar
foo =bar
foo = bar
```

The first line is a valid variable declaration, and will set `foo` equal to the string `bar`

The second line is valid code, but will result in the program `foo=` being run with a single argument `bar`

The third line is valid code, but will result in the program `foo` being run with a single argument `=bar`

The fourth line is valid code, but will result in the program `foo` being run with 2 arguments, `=` and `bar`