tmpdir: Use a good default directory on native Windows.
* lib/tmpdir.c: Include <windows.h>, pathmax.h.
(P_tmpdir): Default to _P_tmpdir on native Windows.
(path_search): On native Windows, try the value returned by GetTempPath
before trying P_tmpdir.
* modules/tmpdir (Depends-on): Add pathmax.
Suggested by John Darrington <john@darrington.wattle.id.au>.