net: import linux socket compatibility layer (not in makefile)

    1. port the linux socket compatibility layer form freebsd
    2. in freebsd, the compatibility is done to support execution of
       32bit ELFs, so I had to rewrite most of the code in linux_socket.c
    3. special note for sendmsg/recvmsg - I put an assert that we don't handle
       ancillary data (msg->msg_control), so calls that are made to these
       functions with msg->msg_control != NULL will fail. if a need emerge
       we can handle this part later. also, the original compatibility layer
       handled only 2 types of messages: credentials and rights,
       which we ignore in OSv.