Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Fix an obvious typo.

MFC after: 3 days

IFp4: Don't rely on disk IDs and always use vdev guids, which means always look
up for components by reading metadata. This might be slower when there are big
number of disks in the system, but is definiately more reliable.

Forced commit to note that these files were repo copied.

Use provider's ident to handle situations when disks are moved around
and show up with different names: first try to open provider using
remembered name and compare its ident, if equal, this is our provider,
if not equal or there is no provider with such name, find provider with
remembered ident and don't care about the name.