LU-6401 uapi: change lustre_cfg.h into a proper UAPI header

LU-6401 uapi: change lustre_cfg.h into a proper UAPI header

Instead of handling memory allocation for both userland and
kernel space remove lustre_cfg_new() and lustre_cfg_free()
and just use what is required for both kernel and user space.
To perserve the truncate warning move lustre_cfg_string()
to obd_config.c. Implement user land version for llog_reader.
Lastly remove libcfs.h via obd_support.h from lustre_cfg.h
and move it to the uapi directory.

Change-Id: I96469b9040a9b3f885e117debba107f90955e09f
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/26966
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Tested-by: Jenkins
Reviewed-by: Ben Evans <bevans@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>