* path.h (path_conv): Make path_flags private. Rename known_suffix to
suffix and make private. Rename normalized_path to posix_path and
make privtae. Accommodate name changes throughout in path_conv
methods.
(path_conv::known_suffix): New method. Use throughout instead of
accessing suffix directly.
(path_conv::get_win32): Constify.
(path_conv::get_posix): New method to read posix_path. Use throughout
instead of accessing normalized_path directly.
(path_conv::set_posix): Rename from set_normalized_path. Accommodate
name change throughout.
* spawn.cc (find_exec): Return POSIX path, not Win32 path.