Handle split AnSiT messages for term.el (Bug#17231)

Handle split AnSiT messages for term.el (Bug#17231)

Check to see if there is an incomplete command at the end of
term-emulate-terminal's input string, and, if so, save it so the whole
command can be processed when the next string arrives.
* lisp/term.el (term-partial-ansi-terminal-message): New variable.
(term-mode): Make it buffer local.
(term-handle-ansi-terminal-messages): Prepend it to the received
message, and set it if a partial message was received.

Copyright-paperwork-exempt: yes

Do not merge to master, it will be solved differently there, see
"Switch term.el to lexical binding, and clean up code a bit".