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/platform.h

@@ -389,6 +389,7 @@ Socket *make_fd_socket(int infd, int outfd, int inerrfd,
 Socket *make_deferred_fd_socket(DeferredSocketOpener *opener,
                                 SockAddr *addr, int port, Plug *plug);
 void setup_fd_socket(Socket *s, int infd, int outfd, int inerrfd);
+void fd_socket_set_psb_prefix(Socket *s, const char *prefix);
 
 /*
  * Default font setting, which can vary depending on NOT_X_WINDOWS.