Add unlink() syscall and /bin/rm.

This patch adds most of the plumbing for working file deletion in Ext2FS.
Directory entries are removed and inode link counts updated.
We don't yet update the inode or block bitmaps, I will do that separately.

VirtualFileSystem/SyntheticFileSystem.h

@@ -62,6 +62,7 @@ private:
     virtual void flush_metadata() override;
     virtual bool write(const ByteBuffer&) override;
     virtual bool add_child(InodeIdentifier child_id, const String& name, byte file_type, int& error) override;
+    virtual bool remove_child(const String& name, int& error) override;
     virtual RetainPtr<Inode> parent() const override;
 
     SynthFS& fs();