Chaining args together inside a single binary would be nice, but it gets nasty
quickly (as I found out adding 3, 4 extra commands). Switch over to using a
git-style exec-ing command where libinput merely changes argv[0] and then
executes whatever it assembled. And those binaries can hide in libexec so they
don't clutter up the global namespace.

This also makes it a lot easier to write man pages, adopt the same style as
git uses.

Compatibilty wrapper scripts are provided for libinput-list-devices and
libinput-debug events. These warn the user about the changed command, then
exec the new one. Expect these wrappers to be removed at some point in the
future.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>