When not logging I/O, use a signal handler that only forwards SIGINT,

When not logging I/O, use a signal handler that only forwards SIGINT,
SIGQUIT and SIGHUP when they are user-generated signals.  Fixes a
race in the non-I/O logging path where the command may receive two
keyboard-generated signals; one from the kernel and one from the
sudo process.

--HG--
branch : 1.8