authcon: Use ktc_GetTokenEx in ClientAuthToken

authcon: Use ktc_GetTokenEx in ClientAuthToken

Use ktc_GetTokenEx, and the related token utilities to check for a
users tokens, rather than using the legacy ktc_GetToken

Export the necessary symbols from libafsauthent on Windows to be able
to do this.

Change-Id: I475587d88d9a480c66b4e8484cc595ea984340a3
Reviewed-on: http://gerrit.openafs.org/2747
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>