NetStation(): Fix overflow error, add eventnoack and flushreadbuffer

On Unix systems, the int32 value for timestamps could overflow because
GetSecs returns time since 1973, not since system bootup. Fix it by
using a timestamp from last sync as baseline.

Add new non-blocking 'eventnoack' command, as counterpart to 'event'.
Add new 'flushreadbuffer' command.

-> Changes contributed by Justin Ales in forum message 18150.
-> This is untested by me, but code looks reasonable. Needs retesting.