[gdb/tui] Don't try deferred curses initialization twice

I noticed that if deferred curses initialization fails, for instance when
using TERM=dumb, and we try the same again, we run into the same error:
...
$ TERM=dumb gdb -batch -ex "tui enable" -ex "tui enable"
Cannot enable the TUI: terminal doesn't support cursor addressing [TERM=dumb]
Cannot enable the TUI: terminal doesn't support cursor addressing [TERM=dumb]
...

I think it's better to try deferred curses initialization only once.

Fix this by changing bool tui_finish_init into a tribool, and using
TRIBOOL_UNKNOWN to represent the "initialization failed" state, such that we
get instead:
...
$ TERM=dumb gdb -batch -ex "tui enable" -ex "tui enable"
Cannot enable the TUI: terminal doesn't support cursor addressing [TERM=dumb]
Cannot enable the TUI
...

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>