Exit gracefully on ctrl-C.

def sigint_handler(signum, frame):
    """Exit gracefully on ctrl-C."""
    global _handled_int
    if not _handled_int:
        _handled_int = True
        if not _env_var_true("_REZ_QUIET_ON_SIG"):
            print >> sys.stderr, "Interrupted by user"
        sigbase_handler(signum, frame)