Kernel: Implement basic support for sys$mmap() with MAP_PRIVATE

You can now mmap a file as private and writable, and the changes you
make will only be visible to you.

This works because internally a MAP_PRIVATE region is backed by a
unique PrivateInodeVMObject instead of using the globally shared
SharedInodeVMObject like we always did before. :^)

Fixes #1045.

Kernel/FileSystem/File.cpp

@@ -54,7 +54,7 @@ int File::ioctl(FileDescription&, unsigned, unsigned)
     return -ENOTTY;
 }
 
-KResultOr<Region*> File::mmap(Process&, FileDescription&, VirtualAddress, size_t, size_t, int)
+KResultOr<Region*> File::mmap(Process&, FileDescription&, VirtualAddress, size_t, size_t, int, bool)
 {
     return KResult(-ENODEV);
 }