croutonxinitrc-wrapper: Make sure children do not ignore signals

Creating a child resets signal handlers, but leaves ignored signals
unchanged (see "man 2 sigaction" for details): replace "" with a
no-op handler "true".

Test:
 - Logout in LXDE works (fixes #851)
 - Ctrl-C in crosh, or sending SIGTERM to xinit both work (exit
   commands are run).