using auxiliary functions because there is no snprintf-variant on
Windows that behaves exactly as specified in the standard:

* Always return the number of characters that would have been printed
  if the size were unlimited (not including the final `\0').
* Always '\0'-terminate the result
* Do not touch the buffer if size=0, only return the number of characters
  that would have been printed. Can be used to estimate the size needed
  and to allocate it dynamically.

See http://www.freebsd.org/cgi/query-pr.cgi?pr=87260 for the reason why
2 ap variables are used.

Approved by:	Heikki