Close standard handles in watchdog subprocesses.

Naturally, there's one really glaring goof I find out instants after
'git push'! If Pageant starts a watchdog subprocess which will wait
until the main process terminates and then clean up the socket, then
it had better not have that subprocess keep the standard I/O handles
open, or else commands like eval $(pageant -X) won't terminate.

I've applied the same fix in the X11 socket creation, though I think
it's less critical there.

unix/ux_x11.c

@@ -159,6 +159,11 @@ int platform_make_x11_server(Plug *plug, const char *progname, int mindisp,
                  * we expect read() to return EOF as soon as
                  * that process terminates.
                  */
+
+                close(0);
+                close(1);
+                close(2);
+
                 setpgid(0, 0);
                 close(cleanup_pipe[1]);
                 close(authfd);

unix/uxagentsock.c

@@ -62,6 +62,11 @@ Socket *platform_make_agent_socket(
                  * we expect read() to return EOF as soon as
                  * that process terminates.
                  */
+
+                close(0);
+                close(1);
+                close(2);
+
                 setpgid(0, 0);
                 close(cleanup_pipe[1]);
                 while (read(cleanup_pipe[0], buf, sizeof(buf)) > 0);