Kernel: Use KBufferBuilder to build ProcFS files and backtraces

This is not perfect as it uses a lot of VM, but since the buffers are
supposed to be temporary it's not super terrible.

This could be improved by giving back the unused VM to the kernel's
RangeAllocator after finishing the buffer building.

Kernel/Process.cpp

@@ -15,6 +15,7 @@
 #include <Kernel/FileSystem/SharedMemory.h>
 #include <Kernel/FileSystem/VirtualFileSystem.h>
 #include <Kernel/IO.h>
+#include <Kernel/KBufferBuilder.h>
 #include <Kernel/KSyms.h>
 #include <Kernel/Multiboot.h>
 #include <Kernel/Net/Socket.h>
@@ -2838,15 +2839,15 @@ int Process::sys$dbgputstr(const u8* characters, int length)
     return 0;
 }
 
-String Process::backtrace(ProcessInspectionHandle& handle) const
+KBuffer Process::backtrace(ProcessInspectionHandle& handle) const
 {
-    StringBuilder builder;
+    KBufferBuilder builder;
     for_each_thread([&](Thread& thread) {
         builder.appendf("Thread %d:\n", thread.tid());
         builder.append(thread.backtrace(handle));
         return IterationDecision::Continue;
     });
-    return builder.to_string();
+    return builder.build();
 }
 
 int Process::sys$set_process_icon(int icon_id)