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()

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>
(cherry picked from commit 959a892afcc1f33871489912a8ea183fc7fca6c2)

Change-Id: Ic9942f08af4ce45310bfffae5922c67a0655774d
Reviewed-on: http://gerrit.openafs.org/3846
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>