pgpool: Allow to compile against gcc 15 (C23).

Allow to compile against gcc 15 (C23).

This commit includes multiple fixes to compile Pgpool-II in Fedora 42,
which uses gcc 15 (C23).

- Modify pool_type.h. "bool" is now standard in C99 and
above. PostgreSQL decided to require C99 to compile it. So we follow
the way, which is just including <stdbool.h>. Also we define
TRUE/FALSE to (bool) 1 and (bool) 0 respectively. They are used only
in Windows build in PostgreSQL but we still use them in some
places. Eventually we should replace it with true/false since we do
not support Windows.

- It is now required that function pointer arguments matches the
function prototype to be called. For example:
static pid_t worker_fork_a_child(ProcessType type, void (*func) (), void *params);
should be:
static pid_t worker_fork_a_child(ProcessType type, void (*func) (void *), void *params);

Also the prototype of pool_create_relcache() is changed,

- raw_expression_tree_walker() calls walker() in many places. Now
callers of walker() should cast the first argument of it using (Node
*). We replace the call:
return walker(((RangeVar *) node)->alias, context);
with:
return WALK(((RangeVar *) node)->alias, context);
where WALK is defined as:
#define WALK(n,c) walker((Node *) (n), c)

- Note: we have lots of warnings regarding OpenSSL while compiling
Pgpool-II in Fedora42. The version used in Fedora42:

$ openssl -version
OpenSSL 3.2.4 11 Feb 2025 (Library: OpenSSL 3.2.4 11 Feb 2025)

The fix is not included in this commit. We need
to look into it in the future.

Discussion: https://github.com/pgpool/pgpool2/issues/124
Backpatch-through: v4.6

Branch
------
V4_6_STABLE

Details
-------
https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=dced45a522995778d11d7b37524c2486b6c13782

Modified Files
--------------
src/include/main/health_check.h | 4 +-
src/include/pool.h | 2 +-
src/include/pool_type.h | 42 ++--
src/include/utils/pool_relcache.h | 10 +-
src/include/utils/pool_select_walker.h | 7 +-
src/main/health_check.c | 37 ++--
src/main/pgpool_main.c | 4 +-
src/rewrite/pool_timestamp.c | 222 ++++++++++-----------
src/streaming_replication/pool_worker_child.c | 2 +-
.../tests/010.rewrite_timestamp/timestamp/main.c | 4 +-
src/utils/pool_relcache.c | 4 +-
11 files changed, 173 insertions(+), 165 deletions(-)