ls: support --hyperlink to output file:// URIs

ls: support --hyperlink to output file:// URIs

Terminals such as iTerm2 and VTE based terminals
(as of version 0.49.1), support hyperlinks when
passed terminals codes as described at:
https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda

* src/ls.c (gobble_file): Allocate an absolute file name to output.
(quote_name): Output the absolute name with the appropriate codes.
(file_escape): A new function to encode file names as per rfc8089.
(main): Handle the new option and call the file_escape_init() helper.
Disable --dired when --hyperlink is specified.
(print_dir): Get the absolute file name here too, so that the
directory name can be linkified.
* NEWS: Mention the new feature.
* tests/ls/hyperlink.sh: Add a new test.
* tests/local.mk: Reference the new test.
* doc/coreutils.texi (ls invocation): Describe --hyperlink.