New facility, platform_start_subprocess.

We already have the ability to start a subprocess and hook it up to a
Socket, for running local proxy commands. Now the same facility is
available as an auxiliary feature, so that a backend can start another
subcommand for a different purpose, and make a separate Socket to
communicate with it.

Just like the local proxy system, this facility captures the
subprocess's stderr, and passes it back to the caller via plug_log. To
make that not look silly, I had to add a system where the "proxy:"
prefix on the usual plug_log messages is reconfigurable, and when you
call platform_start_subprocess(), you get to pass the prefix you want
to use in this case.

unix/fd-socket.c

@@ -371,6 +371,13 @@ void setup_fd_socket(Socket *s, int infd, int outfd, int inerrfd)
     queue_toplevel_callback(fdsocket_connect_success_callback, fds);
 }
 
+void fd_socket_set_psb_prefix(Socket *s, const char *prefix)
+{
+    FdSocket *fds = container_of(s, FdSocket, sock);
+    assert(fds->sock.vt == &FdSocket_sockvt);
+    psb_set_prefix(&fds->psb, prefix);
+}
+
 static FdSocket *make_fd_socket_internal(SockAddr *addr, int port, Plug *plug)
 {
     FdSocket *fds;