Add a couple of socket examples that can be used with WAMR:
- The `timeout_client` and `timeout_server` examples demonstrate socket
send and receive timeouts using the socket options
- The `multicast_client` and `multicast_server` examples demonstrate receiving
multicast packets in WASM
And add several macro controls for `socket_opts` example.
**What**
* Updated `copy_buffer_to_iovec_app` so that it copies as much of the buffer into the iovec as specified
* Throw invalid value when allocating an iovec of size 0
**Why**
* A bug found from TCP client example which allocates 1024 for the iovec size (where the buf size is also 1024) but received bytes is passed in as the `buf_size` argument to `copy_buffer_to_iovec_app`. This would return early after hitting this check `buf + data->buf_len > buf_begin + buf_size`. However, if the amount to copy is less than the iovec size, we should copy that much of the buf size. Eg TCP client sample receives 27(?) bytes at a time, and this copies 27 bytes into the iovec of size 1024
* The TCP client example attempts to recv bytes of size 0, this attempts to wasm malloc size 0, which outputs a warning. We should early return if recv bytes of size 0
Added socket send and recv timeout options with implementation for posix platform.
This is part of a extending support for sockets in WASI. #1336.
Also add sample that sets and reads back the send and receive timeouts using
the native function binding.
