Linux: fix aklog -setpag to work with ktc_SetTokenEx

Linux: fix aklog -setpag to work with ktc_SetTokenEx

The bit of code that allows aklog -setpag to work with recent
linux needed to be moved along with the change from ktc_SetToken
to ktc_SetTokenEx.

While we're in this bit of code, make it depend on the definition
of the syscall in the user space headers instead of relying on a
kernel configure test.

Change-Id: I0e556b514986b5d06daabcff67ecd51b0e4becdd
Reviewed-on: http://gerrit.openafs.org/2976
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>