Fix process startup in pg_rewind.

Fix process startup in pg_rewind.

Don't allow pg_rewind to run as root on Unix platforms, as any new or
replaced files in the data directory would become owned by root. On Windows,
it can run under a user that has Administrator rights, but a restricted
token needs to be used. This is the same we do e.g. in pg_resetxlog.

Also, add missing set_pglocale_pgservice() call, to fix localization.

Michael Paquier and Fujii Masao