MEDIUM: proto: make common fd checks in sock_create_server_socket

MEDIUM: proto: make common fd checks in sock_create_server_socket

quic_connect_server(), tcp_connect_server(), uxst_connect_server() duplicate
same code to check different ERRNOs, that socket() and setns() may return.
They also duplicate some runtime condition checks, applied to the obtained
server socket fd.

So, in order to remove these duplications and to improve code readability,
let's encapsulate socket() and setns() ERRNOs handling in
sock_handle_system_err(). It must be called just before fd's runtime condition
checks, which we also move in sock_create_server_socket by the same reason.