LU-7734 lnet: fix string.h header inclusion

LU-7734 lnet: fix string.h header inclusion

string.h is intended for user space and libcfs_string.h
is intended for kernel space. Use string.h in liblnetconfig
library.

Add cfs_expr_list_values() in the string.h header file since
it's used in liblnetconfig library.

Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: I50b1bb1aff6fe176cfbe28f039f34d063c9265e4
Reviewed-on: http://review.whamcloud.com/21874
Tested-by: Jenkins
Reviewed-by: Olaf Weber <olaf@sgi.com>
Tested-by: Maloo <hpdd-maloo@intel.com>