Merge branch 'illumos'

Merge in ten upstream fixes which have already been made to both
the Illumos and FreeBSD ZFS implementations.  This brings us up
to date with the latest ZFS changes in Illumos.

Credit goes to Martin Matuska of the FreeBSD project for posting
an excellent summary of the upstream patches we were missing.

Illumos #1313: Integer overflow in txg_delay()
Illumos #278:  get rid zfs of python and pyzfs dependencies
Illumos #1043: Recursive zfs snapshot destroy fails
Illumos #883:  ZIL reuse during remount corruption
Illumos #1092: zfs refratio property
Illumos #1051: zfs should handle
Illumos #510:  'zfs get' enhancement - mountpoint as an argument
Illumos #175:  zfs vdev cache consumes excessive memory
Illumos #764:  panic in zfs:dbuf_sync_list
Illumos #xxx:  zdb -vvv broken after zfs diff integration

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #340