Allow multiple crypt-hooks with the same regexp. (closes #3727).

Allow multiple crypt-hooks with the same regexp. (closes #3727).

Changes the crypt-hook to accumulate a LIST of hooks with
the same regexp, as opposed to replacing the hook data.
This is useful for the case of encrypted mailing lists.

Update pgp classic and gpgme to process a LIST of crypt-hook
values instead of just one.

This version of the patch creates a new _mutt_list_hook() function that
(in theory) other hooks could use if they were changed to return a list.
It also changes the behavior when a crypt-hook is declined: previously
it would immediately use the original recipient for key selection.  Now
it will only do that if all crypt-hooks for a recipient are declined.
This allows all, a subset, or none of the hooks to be used.

Thanks to Rejo Zenger, Remco Rijnders, and Dale Woolridge for their work
on various versions of this patch.