see GitHub issue #1

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

`amused show -p' generates a listing in the form of song > current song song ... This adds an heuristic to `amused load' so that the current song can be set if it's prefixed by "> ". It's particularly useful when re-importing the state from a previous run.

this pings all the "interesting" events to clients that issued a IMSG_CTL_MONITOR, except to the one that generated such event.

c->iev.ibuf.pid is our pid, not sure why, but fd is just as well as easier to use.