Check the module path on settings submission

The mimemail module override the smtp_library value when chosen to handle all mail. The problem occurs when the module has already been installed in the all-site modules folder, and you move it or override it by adding it to the per-site folder. The "smtp_library" variable value keep the all-site folder path, and you get a PHP fatal error for the class to be declared twice.

Submitting the mimemail settings again won't update the variable.