Tidies to ${prvscheck: (1) treat missing 3rd argument as an empty

string; (2) reset $prvscheck_address and $prvscheck_keynum at the end,
because their memory gets reclaimed on successful expansion; (3) Tidy
the code for ${prvscheck - it's actually easier than Tom thought :-) and
(4) allow $prvscheck_result to be usable inside the 3rd argument.