Let's get rid of serial_schedule

We've several times discussed doing $SUBJECT by replacing the
makefile's use of serial_schedule with calling parallel_schedule
with --max-connections=1. This'd remove the need to maintain
two lists of regression test scripts.

I got annoyed again just now about how people seem unable to
keep the two lists in the same order, so here is a patch to
get rid of serial_schedule in that way.

(The vcregress.pl changes are untested, but they seem straightforward
enough. I do wonder though why we spell it --max-concurrent-tests
there when the makefile uses --max-connections.)

It'd perhaps be possible to adjust pg_regress so that when
--max-connections=1 its progress output looks exactly the same
as it did with serial_schedule. I doubt it's worth the trouble
though, unless anyone really wants that.

Any objections?

regards, tom lane