LU-6484 build: autoconf checks cleanup

LU-6484 build: autoconf checks cleanup

Some of autoconf checks are not used any more and can be removed.
Some of them should be moved according new place of code.
Rrename LN_CONFIG_CDEBUG to LIBCFS_CONFIG_CDEBUG
fix AC_CHECK_HEADERS([netdb.h]) and AC_CHECK_FUNCS([gethostbyname])
fix caching for check LB_CONFIG_MPITESTS

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I988434d97051972b76b611e7b5a54835e8044734
Reviewed-on: http://review.whamcloud.com/14541
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>