MFC r285261, r285279:

• Fix offset calculation in variable substitution in jail.conf. The following did not work correctly:
  
  A="A_${B}_C_${D}" B="BBBBB" D="DDDD_${E}_FFFFF" E="EEEEE"

• 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.

Approved by: re (gjb)