[2765] Receive and discard data from the fallback socket.
[2765] Gracefully handle socket opening errors in the IfaceMgr.

Also, implemented missing unit tests for the Iface function which opens
v4 sockets in IfaceMgr.
[2765] Improved commentary in the IfaceMgr.
[2765] Invoke error handler if the interface opening failed.
[2765] Fixed error handling in the IfaceMgr::openSockets4.
[2765] Fix an order of parameters passed to the SocketInfo constructor.
[2765] Initialize variable used in unit test to prevent compilation failure
[2765] Pass error handler when opening sockets in the constructor.
[2765] Fixed a typo in the doxygen documentation.
[2765] Updated Developer's guide with the section about raw sockets use.
[2765] Addressed review comments.
[2765] Added throw tag to the openFallbackSocket function.
[2765] Added unit test description in IfaceMgr unit tests.
[2765] Fixed typo in the libdhcp++ developer's guide.