(format_string): Deleted function.
(sexp_vformat): New %t specifier, formatting an optional display
type. Deleted %z specifier. Instead, introduced a new modifier "0"
that can be used with %s, %l and %t, which says that the data is
NUL-terminated.

Rev: src/nettle/sexp-format.c:1.5
Rev: src/nettle/sexp.h:1.11