Add a status message to the Security line when oppenc is enabled.

For each send menu, add the ability to toggle it on or off.
When enabled, the menus won't show the (e)ncrypt or (b)oth options,
and the (c)lear function only clears the SIGN bit when it is active.

Change the gpgme_send_menu() to directly use the ENCRYPT and SIGN
flags instead of the PGPENCRYPT/SIGN and SMIMEENCRYPT/SMIME flags.
Using the latter sometimes resulted in the APPLICATION bit unset, which made
oppenc unhappy.

The send_menus previously used a switch statement using choice numbers.
Since the menus now vary based on the oppenc option and message bit
being set, these were all changed to convert the numbers back to a
choice letter.