User mode proxy should quit after a short timeout

[mcatanzaro]

Since the user mode gssproxy instance is socket-activated, it should quit after a short timeout to reduce system resource utilization, as it will just be activated again later on when needed. Normal timeouts for this would be 1 minute or 5 minutes, depending on how long you prefer. (Maybe using 5 minutes would be better here because it seems to be pretty slow for the first authentication?)