Rename dprintf to DPRINTF and use C99 cpp variadic arguments. (#82)

POSIX specifies a dprintf function that operates on an fd instead of
a stdio stream.  Using upper case for macros is more idiomatic too.
We no longer need to use an extra set of parentheses for debugging
printf statements.
diff --git a/awk.h b/awk.h
index 5a55301..cc30249 100644
--- a/awk.h
+++ b/awk.h
@@ -44,14 +44,13 @@
  */
 #define setptr(ptr, a)	(*(char *)(intptr_t)(ptr)) = (a)
 
-#define	NN(p)	((p) ? (p) : "(null)")	/* guaranteed non-null for dprintf
+#define	NN(p)	((p) ? (p) : "(null)")	/* guaranteed non-null for DPRINTF
 */
 #define	DEBUG
 #ifdef	DEBUG
-			/* uses have to be doubly parenthesized */
-#	define	dprintf(x)	if (dbg) printf x
+#	define	DPRINTF(...)	if (dbg) printf(__VA_ARGS__)
 #else
-#	define	dprintf(x)
+#	define	DPRINTF(...)
 #endif
 
 extern enum compile_states {