Call for review: overriding osrelease and osreldate in jails

[Ian Lepore]

I've added the ability to specify the values returned by sysctl (and
thus by uname) for kern.osrelease and kern.osreldate within a jail. 

The changes are available for review: https://reviews.freebsd.org/D1948

This allows things like running an 8.4 jail on a 10.1 system such that
within the jail the version is reliably spoofed as 8.4.  While the uname
values can be overridden with env vars, the env vars can be wiped out by
scripts that use env(1).  Changing the values returned by sysctl is more
reliable.

-- Ian