hugepages: rework

hugepages: rework

Changed hugepage metric type and type instances.
Made a single entry in types.db for "free" and
    "used" hugepages.
Calculate the used hugepages.
Removed read_syshugepage_dir; replaced with
    walk_directory.
Cleared up descriptions in .pod.
Made the .conf plugin option names less cryptic.
Linux-only plugin; changed configure.ac accordingly.
Stripped function declarations and header includes
    that are not used.
Tidied up globals and made some globals local.
Removed some hardcoded values and fixed spacing.
Other general tidy-ups.

Change-Id: Id0243397d3fd4ac1be90266b266341318c05c903
Signed-off-by: Kim Jones <kim-marie.jones@intel.com>