bug in slnprintf

[Andrew Tridgell]

> > or initialise the whole of the buffer
> > to zero before use.  
> 
> It's not supposed to do that.

right. Initialising the whole buffer is one of the worst aspects of
strncpy() and strncat(). It's very slow.

> Maybe you have a broken vsnprintf that is not returning
> the correct length ? What does your configure
> output say about where the vsnprintf comes from ?
> 
> Are you still using slackware ? Does the libc5 vsnprintf
> return the correct length ?

I think we better add a configure test to see that vsnprintf() is
working right and to use the replacement if not. I suspect the
slackware one is broken.