Tidy up debug support.

1. Make "-d" switch passed to reader.lua enable all debugging
2. Enable debugging for now (development stage) to preserve the current
behaviour. But for the production release I advise to: a) disable it by
not passing "-d" and b) redirect standard output (not just standard
error) of reader.lua to crash.log in kpdf.sh
3. Comment out debug printf()s in pdf.c

reader.lua

@@ -87,6 +87,8 @@ if optarg["h"] then
 end
 
 if not optarg["d"] then
+	Debug = function() end
+	dump = function() end
 	debug = function() end
 end