pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0c6be59f5e34f44b20d9ec3ffb58c1d9a0bf985e

Modified Files
--------------
src/common/file_utils.c         | 77 ++++++++++++++++++++++++++++-------------
src/include/common/file_utils.h |  5 +++
2 files changed, 57 insertions(+), 25 deletions(-)