[krbdev.mit.edu #6097] WIN32 SOCKET != int in krb5 lib

[Ken Raeburn via RT]

The sendauth/recvauth interfaces (and lower-level routines like krb5_write_message) take a 
krb5_pointer argument that's handled as int* internally to find the file descriptor.

1) For Windows, we should be using SOCKET* probably.  (Cf. ticket 6041.)
2) We might want to update the published prototypes to use pointers to real types; the fd 
handle shouldn't be opaque in this interface.

#1 would presumably be an ABI change on 64-bit Windows, but the current version may well be 
unusable, I'm not sure...