conn-ssl.c: work around gnutls API problems on 64 bit platforms

conn-ssl.c: work around gnutls API problems on 64 bit platforms

Alexander Barton reported a compiler warning on 64-bit platforms:
cc1: warnings being treated as errors
conn-ssl.c: In function 'ConnSSL_Init_SSL':
conn-ssl.c:403: error: cast to pointer from integer of
different size

Unfortunately, I couldn't find a real solution; the GNUTLS
API expects 'gnutls_transport_ptr_t' (which is void*),
but the default push/pull functions (send/recv) expect an int.

The only alternative solution is to pass in an address to the
file descriptor, then add send/recv wrappers that expect a pointer.

What a mess[tm].