Handle handle to int casts

• src/engine-assuan.c (handle_to_int): New. (send_options, op_assuan_init, op_assuan_encrypt), (op_assuan_sign, op_assuan_decrypt, op_assuan_verify): Use it.

Doing this in a function explictly ensures that we only downcast
Handle pointers. For them this is allowed and we can locally
deactive the warning.