(uid_t) -1 is still used as placeholder to mean "unset". This is OK, since
there should be no users with such number, see
https://systemd.io/UIDS-GIDS#special-linux-uids.

(uid_t) -1 is used as the default value in class initialization.

When a user or group above INT32_MAX is created, the numeric uid or
gid wraps around to negative when the value is assigned to gint, and
polkit gets confused. Let's accept such gids, except for -1.

A nicer fix would be to change the underlying type to e.g. uint32 to
not have negative values. But this cannot be done without breaking the
API, so likely new functions will have to be added (a
polkit_unix_user_new variant that takes a unsigned, and the same for
_group_new, _set_uid, _get_uid, _set_gid, _get_gid, etc.). This will
require a bigger patch.

Fixes #74.