check return value from read() and write() properly

check return value from read() and write() properly

The system calls read and write may return less than the whole amount
requested for a number of reasons.  So the idioms
   if (read(fd, &object, sizeof(object)) != sizeof(object)) goto fail;
and even worse
   if (read(fd, &object, sizeof(object)) < 0) goto fail;
are wrong.  Additionally, read and write may sometimes return EINTR on
some systems so interruption is not desired or expected a loop is
needed.