It is slow to allocate big buffers using SGX SDK's malloc. Even worse, it
consumes a large amount of precious trusted memory inside enclaves. This
commit avoids using trusted buffers and allocates untrusted buffers for
sendmsg/recvmsg directly via OCall, thus improving the performance of
sendmsg/recvmsg. Note that this optimization does not affect the security of
network data as it has to be sent/received via OCalls.
1. Add a separate net/ directory for the network subsystem;
2. Move some existing socket code to net/;
3. Implement sendmsg/recvmsg with OCalls;
4. Extend client/server test cases.
1. Change the port for server_poll to listen to avoid "address in use" conflict
between test/server and test/server_epoll, and add port as an argument for
test/client to send message
2. As posix-spwan may fail, change the fixed number of processes to spawn to
the number of processes successfully spawned in server_epoll
