Windows specific kWorker/kSubmit construct for avoiding process
creation, caching head files, and keeping tempory files off the disk.
Rewrote kRedirect ($(REDIRECT)) to an internal command, saving an
unnecessary process creation step.
New NT specific directory cache that avoids a lot of stat() calls
and implements invalidation via the internal $(dircache-ctl cmd[,])
function and internal kmk_builtin_dircache command. (The cache
implementation is also used by kWorker.)
Optimized construction of the environment variables for a make job.
