Modify the functions capability_update_inherited_set() and
capability_ambient_set_apply() to drop capabilities not explicitly
requested by the user.

Change test_set_ambient_caps() to test_apply_ambient_caps(), since the
function capability_ambient_set_apply() not only sets ambient
capabilities, but clears inherited capabilities that are not explicitly
requested by the caller.

The function capability_ambient_set_apply() now drops capabilities not
in the capability_ambient_set(), so it is necessary to call it when
the ambient set is empty.

Fixes systemd#13163