For FTP and SFTP based URLs a parameter set to 1 tells the library to list the
names of files in a directory, rather than performing a full directory listing
that would normally include file sizes, dates etc.
For POP3 a parameter of 1 tells the library to list the email message or
messages on the POP3 server. This can be used to change the default behaviour
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.
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.
Setting this option to 1 also implies a directory listing even if the URL
doesnt end with a slash, which otherwise is necessary.
Do NOT use this option if you also use CURLOPT_WILDCARDMATCH(3) as it
will effectively break that feature then.