Provide more item meta data to HTTP RPN.

Working on a few remote-control applications that require more details
about the files in the playlist and the currently playing file. Also
fixed the RPN module so that the fields listed as possible arguments
for vlc_get_meta actually matches the list of fields in the Developer's
wiki.

Signed-off-by: Rémi Denis-Courmont <rdenis@simphalempin.com>