pid_t on mingw

I wanted to propose the attached patch to get rid of the custom pgpid_t
typedef in pg_ctl. Since we liberally use pid_t elsewhere, this seemed
plausible.

However, this patch fails the CompilerWarnings job on Cirrus, because
apparently under mingw, pid_t is "volatile long long int", so all the
printf placeholders mismatch. However, we print pid_t as %d in a lot of
other places, so I'm confused why this fails here.

Also, googling around a bit about this, it seems that mingw might have
changed the pid_t from long long int to int some time ago. Maybe that's
how the pgpid_t came about to begin with. The Cirrus job uses a
cross-compilation environment. I wonder how up to date that is compared
to say the native mingw installations used on the build farm.

Any clues?