a couple readdir syscall changes

* Pre-emptively doing the stat during readdir doesn't seem to save any syscalls, and actually wastes time, so I've removed that.
* Removed database support for stat during readdir: it doesn't return real filenames as part of GetChildOfPath anymore.
* Directories have a correct (arbitrary) timestamp now: set to the time musicfs started up.
* Directories have '.' and '..' entries now. POSIX says these are optional, and everything seems to work okay without them, but it seems right to add them anyway.