Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Some jail parameters (in particular, "ip4" and "ip6" for IP address
restrictions) were found to be inadequately described by a boolean.
Define a new parameter type with three values (disable, new, inherit)
to handle these and future cases.

Approved by: re (kib), bz (mentor)
Discussed with: rwatson

Add libjail, a (somewhat) simpler interface to the jail_set and jail_get
system calls and the security.jail.param sysctls.

Approved by: bz (mentor)