Change "allow_dups" into a flag at hash creation.

Change "allow_dups" into a flag at hash creation.

Instead of having an "allow_dups" parameter for hash_insert(), add a
flag, MUTT_HASH_ALLOW_DUPS, to hash_create().

Currently ReverseAlias, subj_hash, and thread_hash allow duplicate
keys.  Change those hashes to pass the flag at creation, and remove
the last parameter from all callers of hash_insert().