Mailer: discover sendmail path instead of hardcoding it to /usr/sbin/sendmail

`sendmail` can very well be in a path different from
`/usr/sbin/sendmail`.

We already search `$PATH` at `lib/private/Settings/Admin/Mail.php` to
detect whether we want to offer sendmail as a mail transfer method, so
let's be consistent and actually initialize `\Swift_SendmailTransport`
with this path to sendmail, instead of just hardcoding
`/usr/sbin/sendmail`.

Signed-off-by: Florian Klink <flokli@flokli.de>