pefs final report

[Gleb Kurtsou]

Last week I've changed file name checksum algorithm from HMAC to VMAC.
VMAC is much faster and produces 64bit mac and there is no longer need
to truncate HMAC output. I've also changed hand rolled KDF implementation
into HKDF one which use HMAC to derivate keys. Also note that I use mac
algorithms for integrity checks only, there is no authentication
provided by pefs (the same way most of other such filesystems do). I
hope that was a final change of crypto primitives and filesystem format
would remain stable.

Generally, pefs is functional and behaves as it should, all of the
initial project goals were completed. There are some issue to work on
though. Most important one being implementation of file name encryption
cache. Current implementation performs readdir on almost every vnode
lookup not found in iname cache. It degrades performance seriously.