* poll.cc: Add bounds checking for file descriptors. Return POLLNVAL

if fd is invalid. Return POLLERR for each valid fd if cygwin_select
        returned with error.
        include/sys/poll.h: Change POLLERR comment according to above change.

winsup/cygwin/include/sys/poll.h

@@ -18,7 +18,7 @@ __BEGIN_DECLS
 #define POLLIN  1       /* Set if data to read. */
 #define POLLPRI 2       /* Set if urgent data to read. */
 #define POLLOUT 4       /* Set if writing data wouldn't block. */
-#define POLLERR   8     /* An error occured, not used by Cygwin. */
+#define POLLERR   8     /* An error occured. */
 #define POLLHUP  16     /* Shutdown or close happened. */
 #define POLLNVAL 32     /* Invalid file descriptor. */