Introduce variables for initial and max nesting depth on configuration files

The code has been assuming already in a few places that the initial
recursion nesting depth is 0, and the recent changes in hba.c (mainly
783e8c6) have relies on this assumption in more places.  The maximum
recursion nesting level is assumed to be 10 for hba.c and GUCs.

Author: Julien Rouhaud
Discussion: https://postgr.es/m/20221124090724.n7amf5kpdhx6vb76@jrouhaud

src/backend/commands/extension.c

@@ -60,6 +60,7 @@
 #include "tcop/utility.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
+#include "utils/conffiles.h"
 #include "utils/fmgroids.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
@@ -515,7 +516,8 @@ parse_extension_control_file(ExtensionControlFile *control,
 	 * Parse the file content, using GUC's file parsing code.  We need not
 	 * check the return value since any errors will be thrown at ERROR level.
 	 */
-	(void) ParseConfigFp(file, filename, 0, ERROR, &head, &tail);
+	(void) ParseConfigFp(file, filename, CONF_FILE_START_DEPTH, ERROR,
+						 &head, &tail);
 
 	FreeFile(file);