lib/string/sprintf.[ch]: Add [v]snprintf_()

These functions are like [v]snprintf(3), but return -1 on truncation,
which makes it easier to test.  In fact, the API of swprintf(3), which
was invented later than snprintf(3), and is the wide-character version
of it, is identical to this snprintf_().

snprintf(3) is iseful in two cases:

-  We don't care if the output is truncated.  snprintf(3) is fine for
   those, and the return value can be ignored.  But snprintf_() is also
   fine for those.

-  Truncation is bad.  In that case, it's as bad as a hard error (-1)
   from snprintf, so merging both problems into the same error code
   makes it easier to handle errors.  Return the length if no truncation
   so that we can use it if necessary.

Not returning the whole length before truncation makes a better API,
which need not read the entire input, so it's less vulnerable to DoS
attacks when a malicious user controls the input.

Use these functions to implement SNPRINTF().

Cc: Samanta Navarro <ferivoz@riseup.net>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/string/sprintf.c

@@ -17,3 +17,8 @@
 extern inline int xasprintf(char **restrict s, const char *restrict fmt, ...);
 extern inline int xvasprintf(char **restrict s, const char *restrict fmt,
     va_list ap);
+
+extern inline int snprintf_(char *restrict s, size_t size,
+    const char *restrict fmt, ...);
+extern inline int vsnprintf_(char *restrict s, size_t size,
+    const char *restrict fmt, va_list ap);