Syntax check regen/uconfig_h.pl using t/porting/utils.t

It's the only regen script that we can't run as part of the tests (because it
requires a Unix shell), but can syntax check (because it only uses core
modules).

In theory we could make it skip with --tap if $Config{sh} is not what we
expect, but to be robust this looks to be a problem. Firstly, $Config{sh}
can be undef, or something "non-Unix". To be useful a whitelist needs to be
(at least) (?:/usr)?/bin/sh, and potentially also ksh. But the output is not
valid TAP:

$ ./perl -Ilib regen/uconfig_h.pl --tap
Extracting uconfig.h-new (with variable substitutions)
ok - regen/uconfig_h.pl uconfig.h

and t/TEST would choke, so we'd need to capture it or otherwise comment out
that "Extracting" line which just adds both complexity and fragility.

So the right trade off appears to be just to syntax check it.
Refactor t/porting/regen.t to check everything (and the return values!).

Previously it was fire-and-forget for the 3 programs it ran (and for the
programs that regen.pl ran). Now we die if any program fails to return 0.

Also regen.t had an explicit list of programs to test. It turned out that it
was not testing regen/mk_invlists.pl. Now regen.t has a skip list of what
not to test, and everything not skipped it tested. This way any new
additions will not get missed.

This was implemented by refactoring regen.pl to read the list of programs it
runs from <DATA>, so that regen.t can open regen.pl to extract the same
list.