bbc.lua: Fix and simplify URL generation for LL/L3
bbc.lua: Fix and simplify URL generation for LL/L3

The URL can be as simple as $tcurl/$playpath, so generate it after
those 2 items. Also make sure to use the correct separator when
appending the authString to the application.

hadess/libquvi-scripts-iplayer,hadess/libquvi-scripts-iplayer