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/libpq/hba.c

@@ -79,7 +79,7 @@ typedef struct
 /*
  * Memory context holding the list of TokenizedAuthLines when parsing
  * HBA or ident configuration files.  This is created when opening the first
- * file (depth of 0).
+ * file (depth of CONF_FILE_START_DEPTH).
  */
 static MemoryContext tokenize_context = NULL;
 
@@ -620,7 +620,7 @@ free_auth_file(FILE *file, int depth)
 	FreeFile(file);
 
 	/* If this is the last cleanup, remove the tokenization context */
-	if (depth == 0)
+	if (depth == CONF_FILE_START_DEPTH)
 	{
 		MemoryContextDelete(tokenize_context);
 		tokenize_context = NULL;
@@ -650,7 +650,7 @@ open_auth_file(const char *filename, int elevel, int depth,
 	 * avoid dumping core due to stack overflow if an include file loops back
 	 * to itself.  The maximum nesting depth is pretty arbitrary.
 	 */
-	if (depth > 10)
+	if (depth > CONF_FILE_MAX_DEPTH)
 	{
 		ereport(elevel,
 				(errcode_for_file_access(),
@@ -684,7 +684,7 @@ open_auth_file(const char *filename, int elevel, int depth,
 	 * tokenization.  This will be closed with this file when coming back to
 	 * this level of cleanup.
 	 */
-	if (depth == 0)
+	if (depth == CONF_FILE_START_DEPTH)
 	{
 		/*
 		 * A context may be present, but assume that it has been eliminated
@@ -762,7 +762,7 @@ tokenize_auth_file(const char *filename, FILE *file, List **tok_lines,
 
 	initStringInfo(&buf);
 
-	if (depth == 0)
+	if (depth == CONF_FILE_START_DEPTH)
 		*tok_lines = NIL;
 
 	while (!feof(file) && !ferror(file))