file: add support for getting basic directory listings

Not supported on Windows (yet)

Closes #13137

docs/cmdline-opts/list-only.md

@@ -3,10 +3,10 @@ c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 SPDX-License-Identifier: curl
 Long: list-only
 Short: l
-Protocols: FTP POP3 SFTP
+Protocols: FTP POP3 SFTP FILE
 Help: List only mode
 Added: 4.0
-Category: ftp pop3 sftp
+Category: ftp pop3 sftp file
 Multi: boolean
 See-also:
   - quote
@@ -35,6 +35,9 @@ When retrieving a specific email from POP3, this switch forces a LIST command
 to be performed instead of RETR. This is particularly useful if the user wants
 to see if a specific message-id exists on the server and what size it is.
 
+For FILE, this option has no effect yet as directories are always listed in
+this mode.
+
 Note: When combined with --request, this option can be used to send a UIDL
 command instead, so the user may use the email's unique identifier rather than
 its message-id to make the request.

docs/libcurl/opts/CURLOPT_DIRLISTONLY.md

@@ -36,6 +36,9 @@ messages on the POP3 server. This can be used to change the default behavior
 of libcurl, when combined with a URL that contains a message ID, to perform a
 "scan listing" which can then be used to determine the size of an email.
 
+For FILE, this option has no effect yet as directories are always listed in
+this mode.
+
 Note: For FTP this causes a NLST command to be sent to the FTP server. Beware
 that some FTP servers list only files in their response to NLST; they might
 not include subdirectories and symbolic links.