Allow the kern.osrelease and kern.osreldate sysctl values to be set in a
jail's creation parameters. This allows the kernel version to be reliably
spoofed within the jail whether examined directly with sysctl or
indirectly with the uname -r and -K options.

The values can only be set at jail creation time, to eliminate the need
for any locking when accessing the values via sysctl.

The overridden values are inherited by nested jails (unless the config for
the nested jails also overrides the values).

There is no sanity or range checking, other than disallowing an empty
release string or a zero release date, by design. The system
administrator is trusted to set sane values. Setting values that are
newer than the actual running kernel will likely cause compatibility
problems.

Differential Revision: https://reviews.freebsd.org/D1948
Relnotes: yes