(enum sexp_token): New constant SEXP_DISPLAY.

Start constants from 1, to keep 0 free for special uses.
(struct sexp_parse_state): New struct for keeping track of parser
state.
(sexp_parse_init): New function.
(sexp_check_token): New function, replacing sexp_skip_token.
(sexp_parse): New function.
(sexp_convert_item): Simplified by using sexp_parse.
(sexp_convert_list): Use sexp_parse.
(main): Likewise.

Rev: src/nettle/tools/sexp-conv.c:1.7