net: websocket: Implement websocket_recv_msg timeout

Although websocket_recv_msg function accepts timeout parameter, the
functionality was rather limited, allowing only to either work in
non-blocking manner, or to block indefinitely. Any timeout value
other than -1 (forever) ended up in non-blocking operation.

This PR fixes this by implementing a basic timeout mechanism, built on
top of poll(). For now on, only timeout of 0 will result in non-blocking
operation, any other timeout will make the function block for the
specified amount of time.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>