Fix startup hang in Unix file transfer tools.

This seems to be a knock-on effect of my recent reworking of the SSH
code to be based around queues and callbacks. The loop iteration
function in uxsftp.c (ssh_sftp_do_select) would keep going round its
select loop until something had happened on one of its file
descriptors, and then return to the caller in the assumption that the
resulting data might have triggered whatever condition the caller was
waiting for - and if not, then the caller checks, finds nothing
interesting has happened, and resumes looping with no harm done.

But now, when something happens on an fd, it doesn't _synchronously_
trigger the follow-up condition PSFTP was waiting for (which, at
startup time, happens to be back->sendok() starting to return TRUE).
Instead, it schedules a callback, which will schedule a callback,
which ... ends up setting that flag. But by that time, the loop
function has already returned, the caller has found nothing
interesting and resumed looping, and _now_ the interesting thing
happens but it's too late because ssh_sftp_do_select will wait until
the next file descriptor activity before it next returns.

Solution: give run_toplevel_callbacks a return value which says
whether it's actually done something, and if so, return immediately in
case that was the droid the caller was looking for. As it were.

unix/uxsftp.c

@@ -459,6 +459,7 @@ static int ssh_sftp_do_select(int include_stdin, int no_fds_ok)
     int fd, fdstate, rwx, ret, maxfd;
     unsigned long now = GETTICKCOUNT();
     unsigned long next;
+    int done_something = FALSE;
 
     fdlist = NULL;
     fdcount = fdsize = 0;
@@ -509,7 +510,7 @@ static int ssh_sftp_do_select(int include_stdin, int no_fds_ok)
             tv.tv_usec = 0;
             ret = select(maxfd, &rset, &wset, &xset, &tv);
             if (ret == 0)
-                run_toplevel_callbacks();
+                done_something |= run_toplevel_callbacks();
         } else if (run_timers(now, &next)) {
             do {
                 unsigned long then;
@@ -535,7 +536,7 @@ static int ssh_sftp_do_select(int include_stdin, int no_fds_ok)
                 ret = select(maxfd, &rset, &wset, &xset, NULL);
             } while (ret < 0 && errno == EINTR);
         }
-    } while (ret == 0);
+    } while (ret == 0 && !done_something);
 
     if (ret < 0) {
 	perror("select");