Provide mutt_addrlist_append and mutt_addrlist_prepend

Provide mutt_addrlist_append and mutt_addrlist_prepend

These are one-liners that call TAILQ_INSERT_TAIL and TAILQ_INSERT_HEAD,
but the macros have the ugly side-effect of evaluating their arguments
multiple times, so TAILQ_INSERT_TAIL(list, new_element()); doesn't do
what you would expect.