Cygwin: sigproc: Do not send signal to myself if exiting.

- This patch fixes the issue that process sometimes hangs for 60
  seconds with the following scenario.
    1) Open command prompt.
    2) Run "c:\cygwin64\bin\bash -l"
    3) Compipe the following source with mingw compiler.
       /*--- Begin ---*/
       #include <stdio.h>
       int main() {return getchar();}
       /*---- End ----*/
    4) Run "tcsh -c ./a.exe"
    5) Hit Ctrl-C.

winsup/cygwin/sigproc.cc

@@ -603,6 +603,11 @@ sig_send (_pinfo *p, siginfo_t& si, _cygtls *tls)
       its_me = false;
     }
 
+  /* Do not send signal to myself if exiting, which will be
+     ignored in wait_sig thread. */
+  if (its_me && exit_state > ES_EXIT_STARTING && si.si_signo > 0)
+    goto out;
+
   if (its_me)
     sendsig = my_sendsig;
   else