[gdb/testsuite] Initial TERM=ansis support in tuiterm

While investigating using TERM=ansiw for freebsd, I also came across
TERM=ansis which unlike ansiw is present on both linux and freebsd.

Add initial support for TERM=ansi in tuiterm:
- handle ansis in Term::_have_bw
- add Term::_csi_x to support (well, ignore) DECREQTPARM
  (Request Terminal Parameters)

This is sufficient to make the TUI testsuite pass on x86_64-freebsd.