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

Our libc doesn't implement control method for XDR (only kernel does) and it
will always return failure. Fix this by bringing userland implementation of
xdrmem_control() back. This allow 'zpool import' to work again.

Reported by: Thomas Backman <serenity@exscape.org>
Reviewed by: kmacy
Approved by: re (kib)