Don't pass tokens around the backup system

Don't pass tokens around the backup system

The backup system has a global ktc_token, which is used
to work out when its credentials are about to expire. This leads to
an unfortunate dependency throughout the code on the format of this
token.

Replace this with a global time_t which stores the expiry time, and
copy the required field from the token into this when we get the
token. This limits the exposure of the token, and simplifies the code.

Change-Id: Ia2929c2c0a4c1ba9ca5db881865f33af5a732d2f
Reviewed-on: http://gerrit.openafs.org/1218
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>