Cygwin: fhandler_termios::tcsetpgrp: check that argument is non-negative

Return -1 with EINVAL if pgid < 0.

Previously tcsetpgrp() would blindly go ahead and set the pgid of the
controlling terminal to a negative value, causing later calls to
various functions to fail.

For example, gdb has code like the following:

  tcsetpgrp (0, getpgid (inf->pid));

If getpgid (inf->pid) fails (returns -1), then this code would set the
pgid of fd 0 to -1, so that some later calls to getpgid() would also
return -1.  This caused the problem reported here:

  https://cygwin.com/ml/cygwin/2019-07/msg00166.html.

winsup/cygwin/fhandler_termios.cc

@@ -69,6 +69,11 @@ fhandler_termios::tcsetpgrp (const pid_t pgid)
       set_errno (EPERM);
       return -1;
     }
+  else if (pgid < 0)
+    {
+      set_errno (EINVAL);
+      return -1;
+    }
   int res;
   while (1)
     {