xinput: Assume 'list' by default if no args given.

xrandr lists all outputs by default when run with no args.  So, make
xinput list all inputs by default when run the same way, so the tools
are consistent.

Signed-off-by: Bryce Harrington <bryce@canonical.com>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>