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;
 };

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();
+}
+
 }

AK/StringBuilder.cpp

@@ -41,13 +41,18 @@ void StringBuilder::append(char ch)
     m_length += 1;
 }
 
+void StringBuilder::appendvf(const char* fmt, va_list ap)
+{
+    printfInternal([this] (char*&, char ch) {
+        append(ch);
+    }, nullptr, fmt, ap);
+}
+
 void StringBuilder::appendf(const char* fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);
-    printfInternal([this] (char*&, char ch) {
-        append(ch);
-    }, nullptr, fmt, ap);
+    appendvf(fmt, ap);
     va_end(ap);
 }
 

AK/StringBuilder.h

@@ -2,6 +2,7 @@
 
 #include "AKString.h"
 #include "Vector.h"
+#include <LibC/stdarg.h>
 
 namespace AK {
 
@@ -14,6 +15,7 @@ public:
     void append(char);
     void append(const char*, size_t);
     void appendf(const char*, ...);
+    void appendvf(const char*, va_list);
 
     String build();
     ByteBuffer to_byte_buffer();