Improve directory-files{,-recur} options

Improve directory-files{,-recur} options

	Add the following FILES-ONLY options:
	- dirs  to return only directories and symlinks to directories
	- files  to return only files -- but *NOT* symlinks to files
	- symlinks  to return only symlinks -- but *NOT* real files
          or directories.
	Change the following FILES-ONLY options:
	- t  to return only files and symlinks to files
	- subdir  to return only subdirectories -- but *NOT* symlinks to
	  directories, nor the current or parent directories

	This means the behaviour with the following tree:
		tmp/
		tmp/bar -> foo
		tmp/baz/
		tmp/baz/baz/
		tmp/baz/baz/foo/
		tmp/baz/baz/boz -> boz
		tmp/baz/fool -> ../baz/
		tmp/baz/biz
		tmp/baz/booze -> baz/
		tmp/baz/boz -> biz
		tmp/foz -> baz/
		tmp/foo

	The following will yield:
	(directory-files "tmp" nil nil nil t)
	=> ("bar" "foo" "foz")

	(directory-files "tmp" nil nil nil nil)
	=> ("." ".." "bar" "baz" "foo" "foz")

	(directory-files "tmp" nil nil nil 'files)
	=> ("foo")

	(directory-files "tmp" nil nil nil 'dirs)
	("." ".." "baz")

	(directory-files "tmp" nil nil nil 'symlinks)
	=> ("bar" "foz")

	(directory-files "tmp" nil nil nil 'subdir)
	=> ("baz")

	(directory-files-recur "~/tmp" nil nil nil t 10 nil)
	=> ("bar" "baz/baz/boz" "baz/biz" "baz/boz" "baz/fool" "foo")

	(directory-files-recur "~/tmp" nil nil nil t 10 t)
        => ("bar" "baz/baz/boz" "baz/biz" "baz/booze" "baz/boz"
	    "baz/fool" "foo" "foz")

	(directory-files-recur "~/tmp" nil nil nil nil 10 nil)
        => ("bar" "baz" "baz/baz" "baz/baz/boz" "baz/baz/foo" "baz/biz"
	    "baz/booze" "baz/boz" "baz/fool" "foo" "foz")

	(directory-files-recur "~/tmp" nil nil nil nil 10 t)
	=> ("bar" "baz" "baz/baz" "baz/baz/boz" "baz/baz/foo" "baz/biz"
	    "baz/booze" "baz/boz" "baz/fool" "foo" "foz")

	(directory-files-recur "~/tmp" nil nil nil 'files 10 nil)
	=> ("baz/biz" "foo")

	(directory-files-recur "~/tmp" nil nil nil 'dirs 10 t)
	=> ("baz" "baz/baz" "baz/baz/foo")

	(directory-files-recur "~/tmp" nil nil nil 'symlinks 10 nil)
	=> ("bar" "baz/baz/boz" "baz/booze" "baz/boz" "baz/fool" "foz")

	(directory-files-recur "~/tmp" nil nil nil 'subdir 10 t)
	=> ("baz" "baz/baz" "baz/baz/foo")

	* dired.c (TRIVIAL_DIRECTORY_ENTRY): Reorder the strcmp arguments.
	* dired.c: Remove Qsymlink, Qalive_symlink, Qdead_symlink unused
	symbols.  Add new symbols Qsubdir, Qsymlinks, Qfiles, Qdirs to be
	used in directory-files.
	* dired.c (dfr_inner): Determine separately whether a file is a
	symlink or a directory.  If it is a symlink further determine if
	the linked file is a directory.  Include the result according to the
	new options and changed meanings under FILES-ONLY symbol.  Determine
	recursion separately of whether file is directory.  Always avoid
	recursion of the current and parent directories.
	* dired.c (Fdirectory_files): Update DOCSTRING
	* dired.c (Fdirectory_files_recur): Update DOCSTRING
	* dired.c (syms_of_dired): Add new symbols Qsubdir, Qsymlinks,
	Qfiles, Qdirs

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>