MINOR: tools: add strnlen2() helper

strnlen2() is functionally equivalent to strnlen(). Goal is to provide
an alternative to strnlen() which is not portable since it requires
_POSIX_C_SOURCE >= 200809L

include/haproxy/tools.h

@@ -82,6 +82,21 @@
  */
 extern int strlcpy2(char *dst, const char *src, int size);
 
+/*
+ * portable equivalent to POSIX strnlen():
+ * returns the number of bytes in the string pointed to by <s>, excluding
+ * the terminating null byte, but at most <maxlen>. The function does not
+ * look at characters passed <maxlen>.
+ */
+static inline size_t strnlen2(const char *s, size_t maxlen)
+{
+	size_t len;
+
+	for (len = 0; len < maxlen && s[len]; len++)
+		;
+	return len;
+}
+
 /*
  * This function simply returns a locally allocated string containing
  * the ascii representation for number 'n' in decimal.