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.

Juan José Santamaría Flecha

Discussion: https://postgr.es/m/16080-5d7f03222469f717@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f88544904e4b57b1a8e969a5d594288554417c88

Modified Files
--------------
src/bin/pg_ctl/pg_ctl.c | 14 ++++++++++----
src/test/regress/pg_regress.c | 8 +++++++-
2 files changed, 17 insertions(+), 5 deletions(-)