Rewrite S/MIME key searching. Add purpose checking.

Rewrite S/MIME key searching.  Add purpose checking.

Model the smime.c searching off of classic pgp searching.  Create
smime_get_key_by_hash()/addr()/str() functions that use a single
smime_get_candidates() function to search the index.

Use the new smime_keys generated purpose flag in the index to filter by
KEYFLAG_CANENCRYPT and KEYFLAG_CANSIGN.  Old style indices fall back to
all-inclusive behavior.