Add POSIX compat lstat() variant for win32

Add POSIX compat lstat() variant for win32

The existing p_lstat implementation on win32 is not quite POSIX
compliant when setting errno to ENOTDIR.  This adds an option to
make is be compliant so that code (such as checkout) that cares
to have separate behavior for ENOTDIR can use it portably.

This also contains a couple of other minor cleanups in the
posix_w32.c implementations to avoid unnecessary work.