New LUKS spec

http://luks.endorphin.org/spec has an update to the LUKS specification. Changes:

• Added precise AFsplit specification.
• Removed lrw-plain mode spec as the LRW standardization process is not about to be finished any time soon; will be reintroduced when a normative documentation is released by SISWG.
• Extended introduction text.

Thanks to Sarah Dean for providing valuable feedback with respect to the AFsplit specification.