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 66fa6eb.

Discussion: https://www.postgresql.org/message-id/[email protected]

Author: petere