These routines let one compute the intersection of two sets of filecaps
or capability rights, just as filecaps_merge() and cap_rights_merge()
compute the union. This will be useful in an upcoming patch.
filecaps_intersect() is complex due to the need to merge sets of ioctls.
For now this is implemented with a dumb nested loop on the basis that
ioctl lists are typically short enough that this is fine. It may be
better to instead sort the two lists first and step through them
together.
No functional change intended.