Here is a patch that introduces four private function for convert an integer to uid_t/gid_t and back, and consistently apply these functions in all cases of uid_t/gid_t conversions. This should fix this issue and issues 4591, 7365, 15301, and a part of 15989. Also found and fixed some other uid_t/gid_t related bugs.
Please review and test on Windows and Macintosh. uid_t/gid_t not used on Windows, need to check whether I right disable this part of the code. Macintosh has the peculiarities (I found yet one), need to check whether I missed something.
If the patch is good, I will port it to 2.7 and 3.2 (there were many code changes in 3.3).