Import bmake-20160604

Import bmake-20160604

Interesting items from ChangeLog:

          o cached_realpath(): avoid hitting filesystem more than necessary.
          o meta.c: refactor need_meta decision, add knobs for
            missing meta file and filemon data wrt out-of-datedness.
          o meta.c: missing filemon data is only relevant if we read a
            meta file.
            Also do not return oodate for a missing metafile if gn->path
            points to .CURDIR