stream: redo URL parsing, replace m_struct usage with m_config

Move the URL parsing code from m_option.c to stream.c, and simplify it
dramatically. This code originates from times when http code used this,
but now it's just relict from other stream implementations reusing this
code. Remove the unused bits and simplify the rest.

stream_vcd is insane, and the priv struct is different on every
platform, so drop the URL parsing. This means you can't specify a track
anymore, only the device. (Does anyone use stream_vcd? Not like this
couldn't be fixed, but it doesn't seem worth the effort, especially
because it'd require potentially touching platform specific code.)