Refactor the address list generation out of the find_keys routines.

Refactor the address list generation out of the find_keys routines.

All four find_keys routines have a similar set up code for generating a
single address list out of the to, cc, bcc lists.  This patch pulls all
the code into crypt_get_keys.

This is done to simplify the functions before later patches make them
more complicated (with the oppenc_mode parameter).