Unix rtlib: Delay querying terminal size & cursor position until needed

http://www.freebasic.net/forum/viewtopic.php?p=200712#p200712
Unix rtlib: Provide __fb_enable_vt100_escapes global variable

http://www.freebasic.net/forum/viewtopic.php?p=200712#p200712

By default, the rtlib uses some VT100-specific terminal escape sequences
(for querying cursor position and window size and some others) which may
not work on older/other terminals.

This patch adds a __fb_enable_vt100_escapes global variable which is now
checked before these hard-coded VT100 escape sequences are used.
By implementing and setting this global variable to FALSE, FB programs can
disable these escape sequences at runtime, if needed.