Fix sporadic health check failures due to authentication failure.

Fix sporadic health check failures due to authentication failure.

It is reported that health check process fails due to authentication
failures.
[pgpool-general: 9236] Sporadic health check failures due to authentication failure
https://www.pgpool.net/pipermail/pgpool-general/2024-October/001913.html

When health_check_password is empty string, health check process looks
for the password from pool_passwd file. Problem is, the file
descriptor for the file is inherited from parent pgpool process. This
means that pgpool main and health check process (possibly multiple
process) share the same descriptor, which causes various problem
including the issue reported here. To fix the problem, re-open the
file when health check process starts so that each health check
process owns its own file descriptor.

Note that pgpool child process (responsible for frontend sessions)
already re-opens the file descriptor and they are related to the
issue.

Problem reported and analyzed by Emond Papegaaij.
Backpatch-through: v4.1
Discussion: https://www.pgpool.net/pipermail/pgpool-general/2024-October/001913.html