* exceptions.cc (set_signal_mask): Report on input argument rather than

getsigmask.
* fhandler.h (fhandler_base): Make friends with close_all_files.
* pinfo.cc (_pinfo::set_ctty): Add more debugging.
* sigproc.cc (proc_can_be_signalled): Detect state when signal handler thread
is gone in target process as an EPERM situation.  Set errno to ESRCH if process
doesn't exist.
(sigproc_terminate): Set sendsig to illegal value when closed.
(sig_send): Rely on proc_can_be_signalled setting the proper errno.
* syscalls.cc (close_all_files): Detect when all ttys are closed prior to
calling close_all_files.  The ctty needs to be closed explicitly in this case.

winsup/cygwin/fhandler.h

@@ -98,6 +98,7 @@ enum bg_check_types
 class fhandler_base
 {
   friend class dtable;
+  friend void close_all_files ();
  protected:
   DWORD status;
  private: