oppenc_mode is used by crypt_opportunistic_encrypt to determine whether
there are valid keys for all recipients of a message, without prompting
the user.

The patch wraps around prompts, and makes getkeybyaddr methods return a
valid address-matching key without prompting.

The patch also fixes a small problem with gpgme's getkeybyaddr.  When
determining if there were multiple strong matches, it was comparing the
crypt_key_t instead of its kobj member (gpgme_key_t).

The patch also enables a call to crypt_is_numerical_keyid() in
find_keys(), so that crypt-hooks can actually be checked without
prompting when gpgme is enabled.  (The addition was patterned off of the
pgp_findKeys() function).