This is a follow-up to workers/52510.

The patch permits nofork substitutions ("current shell command
substitutions") to access private parameters in the calling function
scope, just as other current shell constructs like brace commands are
able to.  However, private parameters declared within a nofork
substitution are not accessible by nested nofork substitutions.

Also fixed is an existing crash bug on
  explosive { exec {kaboom}>&2 }
  detonator() {
    private kaboom
    explosive
  }

Fixes/adds tests for using "private" along with TYPESET_TO_UNSET, plus
tests and doc for all of the above.