WL#333: Encrypted tables

Implement a low level interface under MyISAM tables so that all data and indexes
are encrypted.

Points to consider
----------------------------
- encrypting only tables is "nothing" if the logs are not
encrypted. Since 5.1 some logs (General log, Slow query log at
least) can be stored in MyISAM tables instead of plain text files, so
this is ok. But the binary log remains: it contains all updates in
clear text: this must be addressed (by storing the binary log into a
MyISAM table? - it would then greatly influence WL#2582 "handler interface to
binlog").
There are also other files which would reveal
important data: temporary files created on slave when replicating LOAD
DATA INFILE. (comment made by Guilhem)
- it _might_ be that an encryption implementation would require
the file to have a structure in pages; but MyISAM stores rows
contiguously in the data file. Maria will however have pages, so it _might_
be easier to implement in Maria than in MyISAM (comment made by Guilhem).