Save instant message history (notifications and telegrams)

This should address issue (feature request) #196.

The last 20 sets of displayed messages are stored as data/msgs/<user-num>.last.#.msg. This number is currently hard-coded, but could be configurable in the future. I say "sets" because messages are batched-up and displayed together normally, unless a user is actively polling for new users (e.g. while at the Ctrl-P/PrivateMsg prompt).

This involved getting rid of some copy/pasta in sbbs_t:getsmsg() as well by creating/using/reusing readsmsg().