make previous patch portable by abstracting to Ndb_check_socket_hup.
Is in portlib rather than mysys due to the socket parameter being NDB_SOCKET_TYPE


ndb/include/portlib/NdbTCP.h:
  Add Ndb_check_socket_hup prototype
ndb/src/common/portlib/NdbTCP.cpp:
  Implement Ndb_check_socket_hup for unix like systems using the poll system call.
ndb/src/common/portlib/win32/NdbTCP.c:
  Implement Ndb_check_socket_hup(NDB_SOCKET_TYPE) for win32 using the select() call.
  
  (should work okay - unable to test on win32 due to status of ndb port though)
ndb/src/mgmapi/mgmapi.cpp:
  Use the portable (portlib) Ndb_check_socket_hup to check socket status