Make enum for sync methods available to frontend code.

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