Since at least MS have something like GetFirstDirEnt() and
GetNextDirEnt() (presumably with superior performance), we
can let MS hackers add support for a dirent walker using
that API instead, while we stick with the posix-style
readdir() calls.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>