Move private declarations shared between guc.c and guc-file.l to new header

Further preparatory refactoring for compiling guc-file.c standalone.

Reviewed by Andres Freund
Discussion: https://www.postgresql.org/message-id/20220810171935.7k5zgnjwqzalzmtm%40awork3.anarazel.de
Discussion: https://www.postgresql.org/message-id/CAFBsxsF8Gc2StS3haXofshHCzqNMRXiSxvQEYGwnFsTmsdwNeg@mail.gmail.com

src/backend/utils/misc/guc-file.l

@@ -15,10 +15,10 @@
 #include <unistd.h>
 
 #include "common/file_utils.h"
+#include "guc_internal.h"
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "storage/fd.h"
-#include "utils/guc.h"
 
 
 /*

src/backend/utils/misc/guc.c

@@ -57,6 +57,7 @@
 #include "commands/variable.h"
 #include "common/string.h"
 #include "funcapi.h"
+#include "guc_internal.h"
 #include "jit/jit.h"
 #include "libpq/auth.h"
 #include "libpq/libpq.h"

src/backend/utils/misc/guc_internal.h

@@ -0,0 +1,26 @@
+/*--------------------------------------------------------------------
+ * guc_internals.h
+ *
+ * Declarations shared between backend/utils/misc/guc.c and
+ * backend/utils/misc/guc-file.l
+ *
+ * Copyright (c) 2000-2022, PostgreSQL Global Development Group
+ *
+ * src/include/utils/guc_internals.h
+ *--------------------------------------------------------------------
+ */
+#ifndef GUC_INTERNALS_H
+#define GUC_INTERNALS_H
+
+#include "utils/guc.h"
+
+extern int	guc_name_compare(const char *namea, const char *nameb);
+extern ConfigVariable *ProcessConfigFileInternal(GucContext context,
+												 bool applySettings, int elevel);
+extern void record_config_file_error(const char *errmsg,
+									 const char *config_file,
+									 int lineno,
+									 ConfigVariable **head_p,
+									 ConfigVariable **tail_p);
+
+#endif							/* GUC_INTERNALS_H */