Remove pgpid_t type, use pid_t instead
It's unclear why a separate type would be needed here. We use plain
pid_t (or int) everywhere else.
(The only relevant platform where pid_t is not int is 64-bit MinGW,
where it is long long int. So defining pid_t as long (which is 32-bit
on Windows), as was done here, doesn't even accommodate that one.)
Reverts
66fa6eba5a61be740a6c07de92c42221fae79e9c.
Discussion: https://www.postgresql.org/message-id/
289c2e45-c7d9-5ce4-7eff-
a9e2a33e1580@enterprisedb.com