jail: consistently populate the KP_JID and KP_NAME parameters

The gaps here, specifically, were:

• When we have to discover a running jail's jid from name, we should populate the missing jid param
• When we populate jid/name from the config, if the name is a jid we wouldn't populate the name; now we do both.
• When we create a jail, we should populate jid and name with whatever details we have now that we didn't both.

As a consequence, we can cleanup a few things:

• vnet.interface and zfs.dataset can just always use the jid
• Trying to populate JNAME should always work now, where it would be a little crashy before if you create a jail that didn't have a name or jid on the command line
• We can simplify the just-prior JID population now that we'll keep a stringified jid in our intparams.

This primarily fixes the below, but the issues with vnet.interface and
zfs.dataset were pre-existing.

Fixes: d8f021add40c3 ("jail: add JID, JNAME and JPATH to env [...]")
Reviewed by: jamie
Differential Revision: https://reviews.freebsd.org/D51502