Include "filemode.h" first, to test the interface.

Include "filemode.h" first, to test the interface.
Assume that filemode.h includes sys/types.h and sys/stat.h.
(HAVE_ST_DM_MODE): New macro, moved here from ls.c.
(ftypelet): Reorder to put common cases first, for efficiency.
Add 'P', 'w'.  Remove 'M', since it's now the caller's responsibility
to do 'M'.
(strmode): Renamed from mode_string, and now stores 12 bytes instead
of 10, for compatibility with FreeBSD.  All callers changed.
(filemodestring): Now stores 12 bytes instead of 10, and sets file types
that can't be deduced solely from st_mode.  First arg is now a const
pointer.