net/socket and net/tcp: Fix a problem in whent there are multiple network devices.  Polls were being sent to all TCP sockets before.  This is not good because it means that packets may sometimes be sent out on the wrong device.  That is inefficient because it   will cause retransmissions and bad performance.  But, worse, when one of the devices is not Ethernet, it will have a different MSS and, as a result, incorrect data transfers can cause crashes.  The fix is to lock into a single device once the MSS is locked locked down.

Networking: Get rid of the tcp_mss macro.  It is confusing and only obfuscates what is really going on

Local loopback:  Set the IFF_UP flags during initialization because the local loopback device is always up

Networking:  Fix network device name assignment logic for the case of the local loopback device.  There is only one local loopback network device and it is unnumbered

Compilation warning fixup

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>

Kconfig: CONFIG_NET_MULTILINK and CONFIG_NET_MULTINIC must be support if the loopback device and any real network device are selected