The lock test creates a lock and checks that it is not stale. This also
tests whether the corresponding process still exists. However, it is
possible that the lock is refreshed concurrently, which updates the lock
timestamp. Calling `processExists()` with a value receiver, however,
creates an unsynchronized copy of this field. Thus call the method using
a pointer receiver.
The helper function uidGidInt used strconv.ParseInt instead of
ParseUint, so it silently ignored some invalid user/group IDs.
Also, improve the error message. "Invalid UID" is more informative than
having "ParseInt" twice (*strconv.NumError displays the function name).
Finally, the user.User struct can be passed by pointer to get reduce
code size.
