Add support for syncfs() in frontend support functions.

Add support for syncfs() in frontend support functions.

This commit adds support for using syncfs() in fsync_pgdata() and
fsync_dir_recurse() (which have been renamed to sync_pgdata() and
sync_dir_recurse()).  Like recovery_init_sync_method,
sync_pgdata() calls syncfs() for the data directory, each
tablespace, and pg_wal (if it is a symlink).  For now, all of the
frontend utilities that use these support functions are hard-coded
to use fsync(), but a follow-up commit will allow specifying
syncfs().

Co-authored-by: Justin Pryzby
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/20210930004340.GM831%40telsasoft.com