Hi,

The attached patch changes vasnprintf.c to avoid using %n in the general
case, ie when the return value of snprintf is usable.  This should help
if more systems decide to make tighten %n usage.  There are plans for
that in OpenBSD land.

The existing comments in vasnprintf.c mention systems where
gl_SNPRINTF_RETVAL_C99 and gl_SNPRINTF_TRUNCATION_C99 pass.  This patch
only considers the usability of the return value of snprintf, as lack
of truncation seems to be a different problem (apparently handled later
in the code).

The patch is kept short so that no copyright assignement is needed, but
further cleanup can be done.  For example the list of systems where %n
is avoided could be shortened.