stack, even if we don't yet print it out.

At some point, vsprintf() needs to be fixed up so that it can correctly
cope with limited-sized buffers (i.e. vsnprintf), long longs, and
standard format specifiers (e.g. "%04x").  We should also remove the
special types (MAC addresses and IP addresses).  This would then enable
us to use gcc's ability to type-check printf format strings.