This is now tested; both multi-file and single-file modes work fine,

This is now tested; both multi-file and single-file modes work fine,
and multi-file is now fully parallelizable.

(flac): Refactor this to encode one wav file to one flac, instead of
looping over all the wavs for a multi-file mode.  This leaves behind
the fa-rip directories instead of cleaning them up; i left a comment
where the cleanup should happen; easy enough to fix.  When creating or
removing files or directories, test for $!{EEXIST} and $!{ENOENT} on
failure rather than stat(2)ing up front.

(flacloop): Move some of the multi-file processing here into our new
Jobs::run get-jobs function; it manages a queue of wav files so we can
fully parallelize even multi-file jobs, instead of having the disc as
the unit of parallelization.