The default pkg(8) from pkg.freebsd.org requires libjail.so,
so mark the jail package as vital along with the runtime
package to avoid errors when libjail.so is removed. This is
a no-op for systems with WITHOUT_JAIL in src.conf(5) and pkg(8)
built from the Ports Collection.
In order to make this work without marking packages such as
the jail-lib32, for example, the jail.ucl file needed to be
split out into separate files similarly to the runtime-*.ucl
files.
Glanced at by: brd
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
9fc3c7f733