P2P: Add mechanism for timing out idle groups

P2P: Add mechanism for timing out idle groups

A new configuration parameter, p2p_group_idle, can now be used to set
idle timeout value for P2P groups in seconds (0 = no timeout). If set,
this values is used to remove P2P group (both GO and P2P client)
interfaces after the group has been idle (no clients/GO seen) for the
configuration duration.

The P2P-GROUP-REMOVED event is now indicating the reason for group
removal when known. For example:
P2P-GROUP-REMOVED wlan0 GO reason=REQUESTED
P2P-GROUP-REMOVED wlan1 client reason=IDLE