Gitlab issue #3 exposed an awkward corner case: if mutt is configured
without PGP or S/MIME, and with GPGME, but $crypt_use_gpgme is unset.
In this case, no backend will be available, but WithCrypto will be set
with both APPLICATION_PGP and APPLICATION_SMIME bits.

That will allow various config vars to enable encryption or signing,
even though there will be no backend available to perform them.  The
message security flag might then be set, but when the user hits send,
will end up back at the compose menu due to the error.

The pgp or smime menu might not even be available to clear the
security setting!

Add a check in send.c before the compose menu is invoked, and give a
warning message for the menu ops inside the compose menu.

I believe this should prevent the issue.  However this is a corner
case combined with user misconfiguration, so I don't believe is worth
a large effort to completely eradicate.