MINOR: connection: remove sock-specific code from conn_sock_send()

MINOR: connection: remove sock-specific code from conn_sock_send()

The send() loop present in this function and the error handling is already
present in raw_sock_from_buf(). Let's rely on it instead and stop touching
the FD from this place. The send flag was changed to use a more agnostic
CO_SFL_*. The name was changed to "conn_ctrl_send()" to remind that it's
meant to be used to send at the lowest level.