Implement crypt_opportunistic_encrypt().

This function will be called to flip encryption on and off based on
message recipients.
Add the crypt_opportunistic_encrypt option and calls.

This patch creates the OPTCRYPTOPPORTUNISTICENCRYPT option and
documentation.

It also adds calls to crypt_opportunistic_encrypt() during initial
message composition, after updating to, cc, or bcc, and after editing
the message (if edit_headers is enabled).