Implement PF_IMMUTABLE flag and apply it to "name" and "jid" in
jail.conf parameters. This flag disallows redefinition of the parameter.

"name" and/or "jid" are automatically defined in jail.conf by using
the jail names at the front of jail parameter definitions. However,
one could override them by using a variable with the same name like
$name = "foo". This confused the parser and could end up with SIGSEGV.

Note that this change also affects a case when all of parameters are
defined in the command line arguments, not in jail.conf. Specifically,
"jail -c name=j1 name=j2" no longer works. This should be harmless.

PR: 196574
Reviewed by: jamie
Differential Revision: https://reviews.freebsd.org/D3017