Apache httpd does not allow you to listen on the same address/port
more than once, which makes abstracting virtual hosts a bit more
cumbersome, as you need to keep track of and listen on addresses
separately from each virtual host.  We here abstract away that, and
allow users to use apache::listen on the same URL multiple times.
Duplicates will then be removed before Apache httpd gets to see the
list of Listen directives.

The implementation now uses concat::file to manage a single include
file containing all the Listen directives, instead of including one
file for each apache::listen resource.  To create a list of unique
Listen directives, we use the 'filter' parameter of concat::file,
running 'sort | uniq'.