selinux: rename comparison functions for clarity

The functions context_cmp(), mls_context_cmp() and ebitmap_cmp() are not
traditional C style compare functions returning -1, 0, and 1 for less
than, equal, and greater than; they only return whether their arguments
are equal.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>