Enable transmission of broadcast packets in UdpSocket

The function allowBroadcast in the UdpSocket implementation sets the respective flag on the socket to enable the transmission of broadcast packets.

Change-Id: I9379b8a3c8870fe1513502f35cacf4b988151e2e
Added List iterator decrement

Change-Id: Ib6d5e98f130ed95fd85a8f0d4ecc6489a47e1804
Remove unnecessary address validity check in UdpSocket
The check for a valid address does not work for broadcast addresses. Checking the return value is sufficient.

Change-Id: Ia4861c2c19c06c002bd0498df0a5f7d0d60851be