Adding the -r argument to xargs makes it not run for no reason when
there are no packages to trim spaces from. Otherwise it'd always run at
least once and sometimes misbehave.
Using the ogg container allows to also embed cover images while still
using the opus codec.
Also fixed an edge case that would make the script fail if the first
thing it iterated on was a regular file which didn't require conversion
if the parent directory wasn't already created.
