htsmsg: missed off the header updates for cleanup
htsmsg: added automatic conversion from string to msg/list

This is done on request if the string can be JSON deserialized. This is
useful for the common API where the webui will be sending in serialized
strings and saves having the special case "args" field.

For things like HTSP, which deal directly in htsmsg, the fields should
already have been converted to the right formats etc...
api webui: simplified webui API by removing "args" field

The API code has also been updated to use the htsmsg auto conversion of
strings to map/list where required. Basic approach is check for list/map
first and if that fails fallback to string (if that's whats appropriate
for a mixed type field).
linuxdvb: updated mux to use new idnode delete routine
idnode: some additional debug
iptv: updated to work with new API

This isn't fully tested and no doubt there are configuration issues to
be resolved. But I was able to receive an RTP stream using negge's server.