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.

Kernel/Process.cpp

@@ -1105,7 +1105,6 @@ int Process::sys$utime(const char* pathname, const Unix::utimbuf* buf)
     }
     inode.set_atime(atime);
     inode.set_mtime(mtime);
-    inode.flush_metadata();
     return 0;
 }