selinux: default_range glblub implementation

A policy developer can now specify glblub as a default_range default and
the computed transition will be the intersection of the mls range of
the two contexts.

The glb (greatest lower bound) lub (lowest upper bound) of a range is calculated
as the greater of the low sensitivities and the lower of the high sensitivities
and the and of each category bitmap.

This can be used by MLS solution developers to compute a context that satisfies,
for example, the range of a network interface and the range of a user logging in.

Some examples are:

User Permitted Range | Network Device Label | Computed Label
---------------------|----------------------|----------------
s0-s1:c0.c12         | s0                   | s0
s0-s1:c0.c12         | s0-s1:c0.c1023       | s0-s1:c0.c12
s0-s4:c0.c512        | s1-s1:c0.c1023       | s1-s1:c0.c512
s0-s15:c0,c2         | s4-s6:c0.c128        | s4-s6:c0,c2
s0-s4                | s2-s6                | s2-s4
s0-s4                | s5-s8                | INVALID
s5-s8                | s0-s4                | INVALID

Signed-off-by: Joshua Brindle <joshua.brindle@crunchydata.com>
[PM: subject lines and checkpatch.pl fixes]
Signed-off-by: Paul Moore <paul@paul-moore.com>