email: Add MAIL_EOL setting to ost-config.php file

This setting allows administrators to add (uncomment) a MAIL_EOL setting in
the ost-config.php config file to define the line ending used for mail
headers and encoded bodies in outbound mail (SMTP, for instance).

By default, CRLF is used by the SMTP email generator as per the RFC 822
standard. However, many administrators can benefit by setting LF (\n) as the
line ending.