FS#1680 - improve email address validation in config plugin

- setting_email and setting_richemail updated to use mail_isvalid() from inc/mail.php
- _pattern improved if any plugin extends either class for its own settings (this maybe
  over cautious. Its probably very unlikely that any plugin does this).

darcs-hash:20090426231346-f07c6-2af83d890ff4d92b14637ef6024d3fb68ba97efd.gz