Add crypt_opportunistic_encrypt option

This PR set implements crypt_opportunistic_encrypt.  This option allows mutt to automatically turn encryption on and off based on whether or not there are keys available for the message recipients.

These changes were not implemented by myself, but are actually patches downloaded from [here](http://www.8t8.us/mutt/patches/). As detailed, they were also submited to [mutt-dev](http://marc.info/?l=mutt-dev&m=136340705622283&w=2), but no response was ever given.

I've built this branch and properly tested this changeset: it all works as expected.