Support O_DIRECTORY flag to open(), for getdents()
Merge a few printk+panic into just panic, since it supports format strings
Fix several bugs, and implement opendir/readdir/closedir for ext2.

* Implement opendir/readdir/closedir for ext2; they use getdents() to actually fetch the data.

Bugs fixed:
* open() failed with -EACCES due to a logic error (OR instead of AND).
* ext2_getdents had a bug with the file type handling, which would result in the file type being DT_UNKNOWN most of the time.
* ext2_getdents had a typo which could cause invalid memory accesses and therefore kernel panics.
Change mount paths and Makefile
Add note to panic() re: backtraces and scrollback
Remove ext2_lsdir, and add ls -i for printing inode numbers in userspace
ext2 cleanup (debug code, TODOs)
Clean up Makefile