pgsql: 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3ed19567198ddb34ab32be6ebdf132148287abdf

Modified Files
--------------
src/backend/storage/file/fd.c       | 4 ++--
src/backend/utils/misc/guc_tables.c | 7 ++++---
src/include/common/file_utils.h     | 6 ++++++
src/include/storage/fd.h            | 6 ------
src/tools/pgindent/typedefs.list    | 1 +
5 files changed, 13 insertions(+), 11 deletions(-)