Add a String::format() and use that in place of ksprintf() in the Kernel.

You're never gonna be right 100% of the time when guessing how much buffer
space you need. This avoids having to make that type of decision in a bunch
of cases. :^)

AK/AKString.h

@@ -95,6 +95,8 @@ public:
 
     ByteBuffer to_byte_buffer() const;
 
+    static String format(const char*, ...);
+
 private:
     RetainPtr<StringImpl> m_impl;
 };