Relative paths for directory listing entries

[joseluisq]

Description

It provides relative path support for directory listing entries (file/directory links) either if --redirect-trailing-slash is enabled or not.

SWS will use relative paths by default regardless of the "redirect trailing slash" feature.
However, when "redirect trailing slash" is disabled and a directory request URI doesn't contain a trailing slash then entries should contain the parent/entry-name as a link format.
Otherwise, SWS will just use the entry-name as a link (default behavior).

Note that in both cases, SWS will append a trailing slash if the entry is a directory.

Motivation and Context

Feature request #136

How Has This Been Tested?

Unit tests on tests/dir_listing.rs that keep relative paths either when --redirect-trailing-slash is enabled or disabled.

Screenshots (if appropriate):

No