vol: use OS_DIRSEP in many more places

vol: use OS_DIRSEP in many more places

For consistency use OS_DIRSEP and OS_DIRSEPC throughout the
package.

Change-Id: I7bb93b4fe7324492e71a6d303e2b74723aaefc5a
Reviewed-on: http://gerrit.openafs.org/3702
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>