QAbstractSocketEngine: introduce QIpPacketHeader for datagrams

QAbstractSocketEngine: introduce QIpPacketHeader for datagrams

This commit changes the readDatagram() and writeDatagram() virtual
functions to take a QIpPacketHeader as meta data, instead of a
QHostAddress/quint16 pair. As previously, the header is an "out"
parameter for readDatagram() and an "in" parameter for writeDatagram().

The header pointer in readDatagram() is allowed to be null if the
PacketHeaderOptions indicates WantNone. Otherwise, it must not be null.
The extra options parameter is introduced because we may not always want
all the metadata upon reception. For sending, we know what to include or
not based on what's set in the incoming header parameter.

QIpPacketHeader splits sender and destination because we'll be able to
return both on datagram reception.

Change-Id: Iee8cbc07c4434ce9b560ffff13ca4213255008c7
Reviewed-by: Richard J. Moore <rich@kde.org>