This takes all of the functions that look up simple data about
paths (such as `git_futils_isdir`) and moves them over to path.h
(becoming `git_path_isdir`).  This leaves fileops.h just with
functions that actually manipulate the filesystem or look at
the file contents in some way.

As part of this, the dir.h header which is really just for win32
support was moved into win32 (with some minor changes).