Add directory-empty-p and new argument COUNT for directory-files-*

Add directory-empty-p and new argument COUNT for directory-files-*

* doc/lispref/files.texi (Contents of Directories): Mention COUNT
argument of directory-files.  Add directory-empty-p.

* etc/NEWS: Mention directory-empty-p and directory-files changes.

* lisp/dired.el (directory-empty-p): New defun.

* lisp/net/ange-ftp.el (ange-ftp-directory-files)
(ange-ftp-directory-files-and-attributes):
* lisp/net/tramp.el (tramp-handle-directory-files)
(tramp-handle-directory-files-and-attributes):
* lisp/net/tramp-adb.el
(tramp-adb-handle-directory-files-and-attributes):
* lisp/net/tramp-rclone.el (tramp-rclone-handle-directory-files):
* lisp/net/tramp-sh.el (tramp-sh-handle-directory-files-and-attributes):
* lisp/net/tramp-smb.el (tramp-smb-handle-directory-files): Add new COUNT
argument.

* src/dired.c (directory_files_internal): Implement new
RETURN_COUNT argument.
(Fdirectory_files, Fdirectory_files_and_attributes): Add new COUNT
argument.

* src/lisp.h (directory_files_internal): Add RETURN_COUNT to declaration.

* src/sysdep.c (list_system_processes): Add Qnil to
directory_files_internal call.

* test/src/dired-tests.el (directory-files-and-attributes-tests):
New file.