Windows: AFSLibExFreePool*() macros

Introduce the AFSLibExFreePool() and AFSLibExFreePoolWithTag() macros
which simply call ExFreePool() and ExFreePoolWithTag().

The prefix AFSLib indicates that memory allocated by
AFSLibExAllocatePoolWithTag() must be freed before unloading.
AFSExFreePool*() cannot be used because that is a pointer to a
function provided by AFSRedir.sys which may not be assigned when
memory must be freed.

The only time that ExFreePool() should be used is if the memory was
allocated by a system function.

Change-Id: I68333d5f2dc907bf7a510f018469a14085ca3a3a
Reviewed-on: http://gerrit.openafs.org/9853
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 4cda20f3d7846b10905645ec915a3ea29bce4f28)
Reviewed-on: http://gerrit.openafs.org/9871