Check gid and uid initalization in PolkitUnixUser and Group objects

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 refuse such uids and gids.

This patch just refuses to initialize uid and gid values to negative.
A nicer fix is to change the underlying type to e.g. gint64 to allow
the full range of values in uid_t and gid_t to be represented. 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
gint64, and the same for _group_new, _set_uid, _get_uid, _set_gid,
_get_gid, etc.). This will require a bigger patch.

Fixes polkit/polkit#74.

Example sessions from uid=4000000000:

Dec 03 14:35:08 krowka polkitd[21432]: system-bus-name::1.41869 is inquiring whether system-bus-name::1.79432 is authorized for org.freedesktop.systemd1.manage-units
Dec 03 14:35:08 krowka polkitd[21432]:  user of caller is unix-user:root
Dec 03 14:35:08 krowka polkitd[21432]: polkit_unix_user_new: assertion 'uid >= 0' failed
Dec 03 14:35:08 krowka polkitd[21432]: polkit_identity_to_string: assertion 'POLKIT_IS_IDENTITY (identity)' failed
Dec 03 14:35:08 krowka polkitd[21432]:  user of subject is (null)
Dec 03 14:35:08 krowka polkitd[21432]: polkit_identity_equal: assertion 'POLKIT_IS_IDENTITY (b)' failed
Dec 03 14:35:08 krowka polkitd[21432]: checking whether system-bus-name::1.79432 is authorized for org.freedesktop.systemd1.manage-units
Dec 03 14:35:08 krowka polkitd[21432]: polkit_unix_user_new: assertion 'uid >= 0' failed
Dec 03 14:35:08 krowka polkitd[21432]:
Dec 03 14:35:08 krowka polkitd[21432]: polkit_authorization_result_get_is_challenge: assertion 'POLKIT_IS_AUTHORIZATION_RESULT (result)' failed
Dec 03 14:35:08 krowka polkitd[21432]: g_object_ref: assertion 'G_IS_OBJECT (object)' failed
Dec 03 14:35:08 krowka polkitd[21432]: g_object_ref: assertion 'G_IS_OBJECT (object)' failed
Dec 03 14:35:08 krowka polkitd[21432]: polkit_authorization_result_get_details: assertion 'POLKIT_IS_AUTHORIZATION_RESULT (result)' failed
Dec 03 14:35:08 krowka polkitd[21432]: polkit_authorization_result_get_is_challenge: assertion 'POLKIT_IS_AUTHORIZATION_RESULT (result)' failed
Dec 03 14:35:08 krowka polkitd[21432]: polkit_authorization_result_get_is_authorized: assertion 'POLKIT_IS_AUTHORIZATION_RESULT (result)' failed
Dec 03 14:35:08 krowka polkitd[21432]: g_object_unref: assertion 'G_IS_OBJECT (object)' failed
Dec 03 14:35:08 krowka polkitd[21432]: g_object_unref: assertion 'G_IS_OBJECT (object)' failed