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/local-proxy.c

@@ -96,3 +96,21 @@ Socket *platform_new_connection(SockAddr *addr, const char *hostname,
         return NULL;
     }
 }
+
+Socket *platform_start_subprocess(const char *cmd, Plug *plug,
+                                  const char *prefix)
+{
+    Socket *socket = make_deferred_fd_socket(
+        null_deferred_socket_opener(),
+        sk_nonamelookup("<local command>"), 0, plug);
+    char *err = platform_setup_local_proxy(socket, cmd);
+    fd_socket_set_psb_prefix(socket, prefix);
+
+    if (err) {
+        sk_close(socket);
+        socket = new_error_socket_fmt(plug, "%s", err);
+        sfree(err);
+    }
+
+    return socket;
+}