bug#32189: 27.0.50; GCC 7 warning due to -Wformat-truncation=2

[Paul Eggert <eggert@cs.ucla.edu>]

On 07/20/2018 12:34 PM, Ken Brown wrote:
> Would it make sense to move its definition to eval.c, where 
> vformat_string is defined, and declare it as external in lisp.h?

Sure, if it's needed. In practice, I've found Fformat and 
Fformat_message to be more useful at the C level (among other things 
they handle more formats), and suggest using them if they're applicable. 
But if they're not, we can use format_string elsewhere -- though it 
would be helpful to note its limitations compared to Fformat_string, in 
a comment perhaps (you can see some of its limitations in the long 
comment at the start of doprnt.c, which the format_string comment could 
refer to).