Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Correct a comment and correct style of a flag check.

MFC after: 3 days

Instead of allocating memory for all the keys at device attach,
create reasonably large cache for the keys that is filled when
needed. The previous version was problematic for very large providers
(hundreds of terabytes or serval petabytes). Every terabyte of data
needs around 256kB for keys. Make the default cache limit big enough
to fit all the keys needed for 4TB providers, which will eat at most
1MB of memory.

MFC after: 2 weeks

Move sc_akeyctx and sc_ivctx initialization to the g_eli_mkey_propagate()
function which eliminates code duplication and will ensure proper order
of operation.

Update copyright years.

MFC after: 1 week

Implement switching of data encryption key every 2^20 blocks.
This ensures the same encryption key won't be used for more than
2^20 blocks (sectors). This will be the default now.

MFC after: 1 week

Remove pointless assignment.

Found with: clang

Implement data integrity verification (data authentication) for geli(8).

Supported by: Wheel Sp. z o.o. (http://www.wheel.pl)

Remove trailing spaces.

Add GEOM_ELI class which provides GEOM providers encryption.
For features list and usage see manual page: geli(8).

Sponsored by: Wheel Sp. z o.o.
http://www.wheel.pl
MFC after: 1 week