Set the 'vital' flag on the runtime and jail packages.

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