Broken writable filesystem

Created by: tedsta

I did several things:

• Implemented File::seek and File::flush in libredox/file.rs
• Implemented sys_lseek and sys_fsync in syscall/call.rs
• Implemented do_sys_fsync and added it as handler for SYS_FSYNC in syscall/handle.rs
• Revised Editor::save in filesystem/apps/editor/editor.rs to write and flush the file.

If you edit a file, then save with F6, then reload with F5, you will see a lovely corrupted string.