Add PF_IMMUTABLE for immutable parameters in jail.conf

[hrs]

Summary

Currently "name" parameter can be redefined as a normal variable
in jail.conf. However, redefinition of it causes a mismatch
between j->intparams[KP_NAME] and j->name, and screws up parameter
configurations. This has been reported in PR 196574.

This patch makes "name" as an immutable parameter. This prevents both

foo { $name = "bar"; }

in jail.conf and a command line like "jail -c name=j1 name=j2".

[jamie]

You may want to do the same with jid. You can give the jail.conf block a numeric tag, which will set the jid instead of the name, but will will still set j->name to that tag string. I would expect the same sort of problem from then setting jid=x inside the block.

[hrs]

Add PF_IMMUTABLE flag to both jid and name

[hrs]

In D3017#59419, @jamie wrote:

You may want to do the same with jid. You can give the jail.conf block a numeric tag, which will set the jid instead of the name, but will will still set j->name to that tag string. I would expect the same sort of problem from then setting jid=x inside the block.

That's true. Thank you for your comment. The updated patch adds immutable flag to jid, too. Can you accept this revision if it is ready to commit?