[BUGFIX] Cleanup Spool Mailer feature

The default file path is not set, so the admin has to set this
to a sensible default outside the web folder when he/she wants
to use it.

Additionally, some CGL fixes and cleanups are applied.

Resolves: #83880
Related: #76349
Releases: master
Change-Id: Ibb3ce564cdfeb474f963c5900fc4bb21e4b7d1a4
Reviewed-on: https://review.typo3.org/55707


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>