Solaris: lookup "" like "."

Solaris: lookup "" like "."

At least on some versions of solaris, we can get passed an empty
string to afs_lookup, if the root directory is in AFS (e.g. after a
chroot). Interpret this as the same as looking up the "." entry;
otherwise we return ENOENT, implying that the "/" directory does not
exist, even if its subdirectories do.

FIXES 127356

Change-Id: I84283e78fbf33b946afaf3c80ef4a1a679e8fc93
Reviewed-on: http://gerrit.openafs.org/2096
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>