sh: Save/restore changed variables in optimized command substitution.

sh: Save/restore changed variables in optimized command substitution.

In optimized command substitution, save and restore any variables
changed by expansions (${var=value} and $((var=assigned))), instead of
trying to determine if an expansion may cause such changes.

If $! is referenced in optimized command substitution, do not cause jobs
to be remembered longer.

This fixes $(jobs $!) again, simplifies the man page and shortens the
code.

Obtained-from:   FreeBSD 223024