Breaking out kern_mac.c into multiple files

[Robert Watson]

As the scope of the MAC Framework has grown, so has kern_mac.c.  It's
reached the point where breaking it into separate files would make it a
lot easier to read, by virtue of logically grouping its exposed functions,
APIs, etc.  Similarly scoped extension frameworks, such as NetGraph and
GEOM, have opted to go into sys/$framework, with files named similarly. 
My leaning was to do something similar -- add sys/mac, and then have
mac_framework.c, mac_net.c, mac_sysvipc.c, etc.  I probably won't get to
this for a bit because I want to avoid introducing large numbers of
conflicts for our outstanding changes, but I was going to poll for general
interest in placement, naming, etc.  Some of the other choices would be to
keep it in kern/, but rename (similar to the System V IPC bits, VFS bits,
et al). 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at fledge.watson.org      Network Associates Laboratories