pgsql: pg_basebackup: Generate valid temporary slot names under PQbacke

pg_basebackup: Generate valid temporary slot names under PQbackendPID()

pgbouncer can cause PQbackendPID() to return negative values due to it
filling be_pid with random bytes (even these days pid_max can only be
set up to 2^22 on 64b machines on Linux, for example, so this cannot
happen with normal PID numbers).  When this happens, pg_basebackup may
generate a temporary slot name that may not be accepted by the parser,
leading to spurious failures, like:
pg_basebackup: error: could not send replication command
ERROR:  replication slot name "pg_basebackup_-1201966863" contains
invalid character

This commit fixes that problem by formatting the result from
PQbackendPID() as an unsigned integer when creating the temporary
replication slot name, so as the invalid character is gone and the
command can be parsed.

Author: Jelte Fennema
Reviewed-by: Daniel Gustafsson, Nishant Sharma
Discussion: https://postgr.es/m/CAGECzQQOGvYfp8ziF4fWQ_o8s2K7ppaoWBQnTmdakn3s-4Z=5g@mail.gmail.com
Backpatch-through: 11

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/dbd1a06c97f5a2f0c13b05cee1d1ca292ed77f7f

Modified Files
--------------
src/bin/pg_basebackup/pg_basebackup.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)