afsd: Fail gracefully on mtab open failure

afsd: Fail gracefully on mtab open failure

On Linux and IRIX, fail gracefully when we fail to open /etc/mtab,
instead of segfaulting. Move strdup'ing cacheMountDir until after
opening /etc/mtab, to simplify the error handling.

Reviewed-on: http://gerrit.openafs.org/4825
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit b1f0bb472e237f5a6f88449db44f030c08a5a324)

Change-Id: Id12f6190eac15593dd32fd46db354e169d19dc2f
Reviewed-on: http://gerrit.openafs.org/6303
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>