Make enum for sync methods available to frontend code.

This commit renames RecoveryInitSyncMethod to DataDirSyncMethod and
moves it to common/file_utils.h.  This is preparatory work for a
follow-up commit that will allow specifying the synchronization
method in frontend utilities such as pg_upgrade and pg_basebackup.

Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/ZN2ZB4afQ2JbR9TA%40paquier.xyz

src/include/common/file_utils.h

@@ -24,6 +24,12 @@ typedef enum PGFileType
 	PGFILETYPE_LNK
 } PGFileType;
 
+typedef enum DataDirSyncMethod
+{
+	DATA_DIR_SYNC_METHOD_FSYNC,
+	DATA_DIR_SYNC_METHOD_SYNCFS
+} DataDirSyncMethod;
+
 struct iovec;					/* avoid including port/pg_iovec.h here */
 
 #ifdef FRONTEND