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