Add seprintf

Based on seprint(2) from Plan 9. I'm not sure if my return value
exactly matches Plan 9's in the case of truncation. seprint(2) is
described only as returning a pointer to the terminating '\0', but
if it does so even in the case of truncation, it is awkward for the
caller to detect. This implementation returns end in the truncation
case, so that (ptr == end) indicates truncation.

chat.h

@@ -44,6 +44,18 @@
 typedef unsigned uint;
 typedef unsigned char byte;
 
+static inline char *seprintf(char *ptr, char *end, const char *fmt, ...)
+	__attribute__((format(printf, 3, 4)));
+static inline char *seprintf(char *ptr, char *end, const char *fmt, ...) {
+	va_list ap;
+	va_start(ap, fmt);
+	int n = vsnprintf(ptr, end - ptr, fmt, ap);
+	va_end(ap);
+	if (n < 0) return NULL;
+	ptr += n;
+	return (ptr > end ? end : ptr);
+}
+
 struct Cat {
 	char *buf;
 	size_t cap;