Change entry qualifier set/get behaviour

Change entry qualifier set/get behaviour

This was intended to address #13, but investigation found out more
breakage than just that. It's hard to make overflow/underflow tests
without assuming the signedness of the uid_t/gid_t types, so
assume/require that they're unsigned (it is true with glibc, MacOS and
FreeBSD) and use this to improve the behaviour:

- Fix setting very large qualifiers, both in the sense of correctly
  reporting overflow when too large, and not longer falsely reporting
  overflow for larger than signed max but smaller than unsigned max;
- Fix returning very large (larger than signed max value) qualifiers;

Fixes #13.