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/String.cpp

@@ -1,5 +1,7 @@
 #include "AKString.h"
 #include "StdLibExtras.h"
+#include "StringBuilder.h"
+#include <LibC/stdarg.h>
 
 namespace AK {
 
@@ -92,4 +94,14 @@ unsigned String::toUInt(bool& ok) const
     return value;
 }
 
+String String::format(const char* fmt, ...)
+{
+    StringBuilder builder;
+    va_list ap;
+    va_start(ap, fmt);
+    builder.appendvf(fmt, ap);
+    va_end(ap);
+    return builder.build();
+}
+
 }