Add sync() syscall and a /bin/sync.

It walks all the live Inode objects and flushes pending metadata changes
wherever needed.

This could be optimized by keeping a separate list of dirty Inodes,
but let's not get ahead of ourselves.

VirtualFileSystem/Ext2FileSystem.cpp

@@ -306,6 +306,7 @@ void Ext2FSInode::populate_metadata() const
 
 void Ext2FSInode::flush_metadata()
 {
+    dbgprintf("Ext2FSInode: flush_metadata for inode %u\n", index());
     m_raw_inode.i_size = m_metadata.size;
     m_raw_inode.i_mode = m_metadata.mode;
     m_raw_inode.i_uid = m_metadata.uid;