LU-6245 libcfs: remove prim wrappers for libcfs

LU-6245 libcfs: remove prim wrappers for libcfs

The libcfs layer contains wrappers to handle primitive
management for both kernel space and user land. Since
libcfs is no longer built for user land we can remove
the wrappers. For linux-prim.h we kept only what is
needed to deal with different kernel versions. Also
we separated out libcfs_string.h to util/string.h so
we can remove the wrappers no longer needed.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I0bad836f20ef29bf2ed17f1f4e5e1234d193868e
Reviewed-on: http://review.whamcloud.com/13874
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>