Fix two bugs in the limit of large numbers of sockets:

Fix two bugs in the limit of large numbers of sockets:

* Fix the maximum number of sockets allowed in the session to 2048,
  instead of making it relative to sizeof(wsa_event).

  The original choice of 2048 was in order to fit the wsa_events array
  in the .cygwin_dll_common shared section, but there is still enough
  room to grow there to have 2048 sockets on 64-bit as well.

* Return an error and set errno=ENOBUF if a socket can't be created
  due to this limit being reached.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>