* osutil/sys: reimplement getuid and chown with the right int type
Due to golang#22739, os.Getuid() and co, and os.Chown and co, use the
wrong integer type and are thus wrong for high ids (especially
noticeable on 32-bit arches). This change addresses this (the only
remaining uses are for comparing with 0, which is OK), but it still
impacts godbus in our dependencies -- anything that uses godbus on a
system with high uids is broken today.
* osutil/sys: use a custom type for uids and gids
* cmd/snap-update-ns: also switch to uint32 uids/gids
* bboozzoo finds a bug, leads me to write a test, which finds a deeper bug
* in 14.04 tests go is not on the default PATH...