vol: avoid double dir separators from addtoname

vol: avoid double dir separators from addtoname

The manner in which addtoname() is used can often result
in double dir separators in path names.  Check for it and
avoid doubling the separator in addtoname()

Change-Id: Ie33459bec3bcc93f81f13020cd35ed8c473b6ca5
Reviewed-on: http://gerrit.openafs.org/3699
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>