* cygtls.h (exitsock): New element.

(exitsock_sin): Ditto.
* cygtls.cc (_cygtls::init_thread): Initialize exitsock to invalid handle.
(_cygtls::call2): Close exitsock if it is valid.
* select.cc (struct socketinf): Remove sin element.
(start_thread_socket): Initialize one SOCK_DGRAM socket per thread instead of
(apparently) expensive opening and closing of socket with each select call.
(socket_cleanup): Send a byte to the exitsock socket as a way to potentially
signal a waiting-for-socket thread to exit.
* tlsoffsets.h: Regenerate.

winsup/cygwin/cygtls.h

@@ -16,6 +16,8 @@ details. */
 #define _NOMNTENT_FUNCS
 #include <mntent.h>
 #undef _NOMNTENT_FUNCS
+#define USE_SYS_TYPES_FD_SET
+#include <winsock.h>
 
 #define CYGTLS_INITIALIZED 0x43227
 #define CYGTLS_EXCEPTION (0x43227 + true)
@@ -61,6 +63,10 @@ struct _local_storage
   char mnt_fsname[CYG_MAX_PATH];
   char mnt_dir[CYG_MAX_PATH];
 
+  /* select.cc */
+  SOCKET exitsock;
+  struct sockaddr_in exitsock_sin;
+
   /* strerror */
   char strerror_buf[20];