Don't require that the maximum length of a file name

Don't require that the maximum length of a file name
encountered in a traversal fit in an `unsigned short',
and fix some portability bugs (don't depend on gcc).

Include "fts_.h", not <fts.h>.
(ALIGNBYTES) [!(__GNUC__ >= 2)]: Add a definition that works with
compilers that don't have __alignof__.
(MAX): Use a definition that doesn't depend on gcc.
(fts_build): Make `len' and `maxlen' be of type size_t, not int.
Test for overflow in a less type-dependent manner.
Test HAVE_STRUCT_DIRENT_D_TYPE, rather than
defined DT_DIR && defined _DIRENT_HAVE_D_TYPE.
(fts_palloc): Test for overflow in a less type-dependent manner.
(fts_safe_changedir): Use stat, not stat64.
Use fstat, not __fxstat64(_STAT_VER.