Minimise rewriting of /etc/gettytab

See forum http://forum.pfsense.org/index.php/topic,57325.0.html
Avoid possible problems with having a partial /etc/gettytab file by not rewriting it at every boot.
Also, I logged to syslog when the setting is changed and gettytab is rewritten. I think that is worth doing when the user changes the setting in System:Advanced:Console Options.