Provide helper for retrying partial vectored I/O.

Provide helper for retrying partial vectored I/O.

compute_remaining_iovec() is a re-usable routine for retrying after
pg_readv() or pg_writev() reports a short transfer.  This will gain new
users in a later commit, but can already replace the open-coded
equivalent code in the existing pg_pwritev_with_retry() function.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com