Add JS console.ansi_getdims()

Use sbbs_t::getdimensions() or JS console.getdimensions() to move user
cols/rows values to run-time console values (querying ANSI terminal if
appropriate/supported).

JS console.pushxy(), popxy(), and gotoxy() all return Boolean now.

sbbs_t::getdimensions() and its JS wrapper is now the proper way to propagate
user's cols/rows settings to the run-time console values. This was done
(post-login) only via use of the TERMROWS and TERMCOLS @-codes in
user_settings.js. Weird.