ugtrain: only check absolute path if '-P' is given

We only need the absolute game path for running the game. So
neither try to get it nor fail if it's not set with pure static
memory configs without '-P', '-D' or '-A' option given.

Fixes a regression introduced by commit e11f22ea
"ugtrain: add 'game_path' config command"
getch_l: restore terminal upon SIGINT

From time to time atexit() isn't enough to restore the terminal.
This is especially the case if SIGINT is used. So use a signal
handler additionally for restoring the terminal upon SIGINT.