Add a function to check for active connections.

Add a function to check for active connections.

If a connection sends data in one way, then the receiver will have shut
down data in the other way, and when the sender is finished he will also
shut down his direction, so the connection looks closed to the sender,
but the receiver might actually still miss the final packets. So UTCP
should keep running until the receiver has received a FINACK and is in the
TIME_WAIT state.

We consider UTCP to be active when there is at least one connection not in
the CLOSED or TIME_WAIT state.

The test program now uses this condition, which allows a transfer of a file
to complete without missing the last few bytes.