Add new debug option noSaveBeforeDecrypt

With GpgOL 2.5.7 we always save a mail before decrypting
it to let all changes made to the mail by other plugins or
and by the user to be saved before we insert the plaintext and
where we afterwards block all writes with the plaintext back
to the client. There have been some reports that this may have
unintended side effects for some users. In which case a new
debug option will allow them to try if the old behavior worked
better for them and then enable that again.

There was little feedback of problems with this behavior so the
default stays that the mail is saved but it is better to be on
the safe side and have the fallback as an option available.

• GnuPG-Bug-Id: T6676