Standardize format for printing PIDs

Standardize format for printing PIDs

Most code prints PIDs as %d, but some code tried to print them as long
or unsigned long.  While this is in theory allowed, the fact that PIDs
fit into int is deeply baked into all PostgreSQL code, so these random
deviations don't accomplish anything except confusion.

Note that we still need casts from pid_t to int, because on 64-bit
MinGW, pid_t is long long int.  (But per above, actually supporting
that range in PostgreSQL code would be major surgery and probably not
useful.)

Discussion: https://www.postgresql.org/message-id/289c2e45-c7d9-5ce4-7eff-a9e2a33e1580@enterprisedb.com