Big performance improvement for fts-based tools that use FTS_NOSTAT.

Big performance improvement for fts-based tools that use FTS_NOSTAT.
Avoid spurious inode-mismatch problems on non-POSIX file systems.
Details: http://article.gmane.org/gmane.comp.lib.gnulib.bugs/7416
* lib/fts_.h (FTS_DEFER_STAT): Define new flag.
(FTS_OPTIONMASK): Extend the mask to reflect this addition.
* lib/fts.c (DT_IS_KNOWN, DT_MUST_BE): Define.
(FTS_NO_STAT_REQUIRED, FTS_STAT_REQUIRED): Define.
(fts_set_stat_required): New function.
(fts_open): Defer the calls to fts_stat, if possible or requested.
Move the code that maps a command-line fts_info value FTS_DOT to FTS_D
into fts_stat itself.
(fts_read): Perform any required (deferred) fts_stat call.
(fts_build): Likewise, for the directory we're about to open and read.
In the readdir loop, carefully decide whether each entry will require
an eventual call to fts_stat, using dirent.d_type info if available.
(fts_stat): Move the test for whether to honor FTS_COMFOLLOW on
a command line argument into this function.  Update all callers.
Map a return value of FTS_DOT to FTS_D for a command line argument.
* modules/fts (Depends-on): Add d-type.  Alphabetize.
Thanks to Miklos Szeredi for his tenacity and for the initial
bug report about "find" failing on a FUSE-based file system.