Show info when gpgol is in conflict with ol

* src/mail.cpp (Mail::prepareCrypto_o): Check the
Outlook crypto flags and show warning.
* src/mail.h: Update accordingly.
* src/mailitem-events.cpp (SEND): Abort when
prepareCrypto_o returns a value.

This improves the case where GpgOL has S/MIME disabled
but outlooks internal S/MIME is used. In that
case we would previously have undefined behavior, now
we either abort the operation or overwrite outlooks
settings.

This helps but does not solve:

• GnuPG-Bug-Id: T4953