Initial implementation of the HTree directory index.

This is a port of NetBSD's GSoC 2012 Ext3 HTree directory indexing
by Vyacheslav Matyushin.  It was cleaned up and enhanced for FreeBSD
by Zheng Liu (lz@).

This is an excellent example of work shared among different projects:
Vyacheslav was able to look at an early prototype from Zheng Liu who
was also able to check the code from Haiku (with permission).

As in linux, the feature is not available by default and must be
enabled explicitly with tune2fs. We still do not support the
workarounds required in readdir for NFS.

Submitted by:	Zheng Liu
Tested by:	Mike Ma
Sponsored by:	Google Inc.
MFC after:	1 week