scanner: Fix reported executable name to 'wayland-scanner'

'wayland-scanner -v' (correctly) reports the program as named
"wayland-scanner", but 'wayland-scanner -h' was inconsistent, referring
to it as './scanner'.

Also refactor this and other references to the program name to use a
common #define, PROGRAM_NAME.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Tested-by: Yong Bakos <ybakos@humanoriented.com>