auth: check for keytab before using in akimpersonate

Heimdal 1.5.2 happily tries to dereference a keytab file even if
none is there. if we have a FILE: type, stat it before use

1.6 only change: master uses KeyFileExt

FIXES 131852

Change-Id: I939eb9e47d2dbbef75c2a64724fdb9111f600150
Reviewed-on: http://gerrit.openafs.org/11075
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>