interface for decoding.
(sexp_input_start_coding): New function.
(sexp_input_end_coding): New function.
(sexp_get_base64_string): Rewrote to use sexp_input_start_coding
and sexp_input_end_coding.
(sexp_get_token): Generate SEXP_TRANSPORT_START tokens.
(sexp_convert_list): Lists are ended only by SEXP_LIST_END.
(sexp_convert_item): Implemented transport mode, using
sexp_input_start_coding and sexp_input_end_coding.

Rev: src/nettle/examples/sexp-conv.c:1.11