pgsql: On Windows, use COMSPEC to find the location of cmd.exe.
On Windows, use COMSPEC to find the location of cmd.exe.
Historically, psql consulted COMSPEC to spawn a shell in its \! command,
but we just invoked "cmd" when spawning shells in pg_ctl and pg_regress.
It seems better to rely on the environment variable, if it's set,
in all cases.
It's debatable whether this is a bug fix or just a behavioral change,
so no back-patch.