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.
This version is for RELENG_2_0 branch.
Tested on Alix nanobsd system running 2.0.2
Merge pull request #305 from phil-davis/RELENG_2_0

Minimise rewriting of /etc/gettytab