(sexp_format): Return length of output. Allow

buffer == NULL, and onyl compute the needed length in this case.
Renamed %s to %z. New format specifiers %s, %i, and %l.
(sexp_vformat): New function.
(format_prefix): Rewrote to not use snprintf.

Rev: src/nettle/sexp-format.c:1.2
Rev: src/nettle/sexp.h:1.7