Add spamfilter::rule (preconditions), add context to crule parser,

and add the first functions: online_time() and reputation().

The more interesting stuff will follow later...

src/misc.c

@@ -971,7 +971,7 @@ char *spamfilter_target_inttostring(int v)
 		if (v & e->value)
 			*p++ = e->character;
 	*p = '\0';
-	return buf;
+	return *buf ? buf : "-";
 }
 
 /** Replace underscores back to the space character.