[#260,!120] Extracted IfaceMgr thread bits into new class, isc::dhcp::Receiver

[#260,!120] Extracted IfaceMgr thread bits into new class, isc::dhcp::Receiver

src/lib/dhcp/iface_mgr.*
    Extracted the thread, watch sockets, mutex, and error message members
    from IfaceMgr and wrapped them in a new class, isc:dhcp::Receiver.

    IfaceMgr::add_fd(int fd, int& maxfd, fd_set* sockets) - new
    convenience method for adding descriptors to fd_set(s)

src/lib/dhcp/tests/iface_mgr_unittest.cc
    class ReceiverTest - new test fixture
    TEST_F(ReceiverTest, receiverClassBasics)  - new unit test