Add a simple StringBuilder::appendf() and use it for ProcFS.

Okay, now ProcFS doesn't crash due to the crappy buffer size estimates
not really working out. This thing has dogshit performance and I will
fix that separately.

AK/StringBuilder.h

@@ -13,6 +13,7 @@ public:
     void append(const String&);
     void append(String&&);
     void append(char);
+    void appendf(const char*, ...);
 
     String build();