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.

Change-Id: I214d8c762573b308d79fa7122a91d48ddd403eb9
Reviewed-on: http://gerrit.openafs.org/7540
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>