afsd: check if mountdir is an absolute path

if the mountdir in the cacheinfo file is not absolute,
it can confuse commands like "df". Thus, force it to
be absolute.

Reviewed-on: http://gerrit.openafs.org/10250
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit fb623d5da30e6d304f672d8485da2e1781afa5df)

Change-Id: I80542020848c901e57e7440cb1ed2149aa0d3251
Reviewed-on: http://gerrit.openafs.org/10253
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>