LibAAH_RTP: Refactor TXGroup code, add unicast mode.

Significantly refactor the TXGroup code to allow transmit groups to
operate in a unicast fanout mode in addition to the traditional pure
multicast mode.  Important changes include...

+ Each transmit group active in the system now has its own socket to
  send and receive traffic on.  In the past, this socket was used to
  listen for retry requests from clients.  Now it is also used to
  listen for group membership reports (IGMPv3 style) from unicast
  clients.  Having an individual socket per transmit group allows
  unicast clients to join the group needing only the IP address and
  port of the transmitters socket, and not needing any additional
  "group id" to be sent to the client beforehand.
+ Setup for the transmitter is now slightly different.  As before, to
  setup for multicast mode, a user can call setRetransmitEndpoint
  passing an IPv4 multicast address and specific port to transmit to.
  It used to also be the case that a user could pass a specific
  unicast address and port to transmit to as well.  This is no longer
  allowed.  Instead, to operate in unicast mode, a user passes 0.0.0.0
  (IPADDR_ANY) as the IP address.  In addition, they need to pass
  either 0 for a port to create a new unicast mode TX group, or they
  need to pass a specific port to cause the player to attempt to use
  an existing unicast mode TX group.  The specific port should be the
  command and control port of the TX group which was bound to when the
  group was originally created.
+ A magic invoke was added to allow clients to fetch the command and
  control port on which a TX Player's TX Group is listening.

The API described above is most likely temporary and should eventually
be replaced with one where TX groups are formal top level objects with
their own independent interface and life-cycle management.

Signed-off-by: John Grossman <johngro@google.com>
Change-Id: Ib4e9737c10660d36c50f1825c9824fff5390b1c7