Add a security bit to the message for oppenc mode.

Add a security bit to the message for oppenc mode.

This allows oppenc to be enabled/disabled on a message level.  If
something initially enables encryption, such as crypt_autoencrypt or
crypt_replyencrypt, oppenc is turned off for the message.

Change the postpone/resume code to persist the oppenc bit.

Also change resend message to enable and invoke oppenc if the option is
set.