This adds the internal management of the seeking, as well as the decoder backends bits.

match the struct sio_par

makes the player process optionally send back a string describing the failure to provide a better and more coincise error message. This turns the syslog message(s): amused: unknown file type amused: failed to play, skipping /home/op/pics/phos.jpg into: amused: unknown file type; skipping /home/op/pics/phos.jpg

avoids a sio_stop in the common case of switching from song that needs similar params (such as tracks in the same album.)

was the only function to disrespect the player_* namespace (well, excepting play). while here also don't mark those two as static, they were the only static functions in the file. I'm not trying to enforcing private symbols here.