Clean up write() return type

Clean up write() return type

and analogously for pg_pwrite() and FileWrite()

Be sure to store the return value in a variable of type ssize_t, not
int.

Some callers of FileWrite() did not have a separate error message for
a short write.  This is okay in practice because FileWriteV() sets
ENOSPC for all non-error returns, so you'll get a reasonable error
message either way.  But callers handled this inconsistently, and this
behavior isn't really prominently documented and commit 871fe4917e1
seems to frown upon it, so it seems better to make all callers handle
this consistently by adding the separate error message.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/f9aab072-0078-49e4-ab93-3b08086a4406@eisentraut.org