libafs: fs flushall for unix cm

Implement the fs flushall command on the unix cache manager to flush
all volume data.  Uses a new common pioctl code point VIOC_FLUSHALL (14),
registered with the grand.central.org assigned numbers.

Reviewed-on: http://gerrit.openafs.org/7540
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 4197bbecd9d0b2ff0b8eaec75a0df9a64f713cf0)

Change-Id: Ia4e9eb070d279bf05dbe38ea9b99f1f2f6b9c6f0
Reviewed-on: http://gerrit.openafs.org/9390
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>