s4:librpc: make all but dcerpc_pipe->binding_handle internal struct members

s4:librpc: make all but dcerpc_pipe->binding_handle internal struct members

We could use a dcerpc_internal.h for struct dcecli_security and
struct dcecli_connection, but in struct dcerpc_pipe we still
expose binding_handle and changing that would require way too
much work for now...

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>