handling of comments.

* tools/sexp-conv.c (sexp_convert_item): Use sexp_put_soft_newline
to terminate comments, and modify indentation for the case that a
list starts with a comment.

* tools/output.c (sexp_output_init): Initialize soft_newline.
(sexp_put_raw_char): Clear soft_newline.
(sexp_put_newline): Check and reset soft_newline.
(sexp_put_soft_newline): New function.

* tools/output.h (struct sexp_output): Removed union with single
element, and updated all users. New attribute soft_newline.

Rev: nettle/ChangeLog:1.31
Rev: nettle/testsuite/sexp-conv-test:1.3
Rev: nettle/tools/output.c:1.3
Rev: nettle/tools/output.h:1.3
Rev: nettle/tools/sexp-conv.c:1.4