(find_mh_folder): Take %headers as arguments, don't call get_headers

(find_mh_folder): Take %headers as arguments, don't call get_headers
or log_headers here.

(filter_mail): .mincfilters, mincspam, and .minchooks are now just one
file, .minc.  Now we don't magically know the name of some
spam_start_hook, spam_stop_hook, spam_check, and post_store_hook, but
instead run all the hooks in the @start_hooks, @stop_hooks,
@filter_hooks, and @post_store_hooks lists.  Call get_headers and
log_headers here so we can pass them to the hooks.