reimplement `restart' on top of seek

use (a modified version of) kristaps' oconfigure see GitHub issue #1

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

declare local functions as static

keep track of current position and total duration

improving error reporting 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

skip every non-regular file not only directories... I guess we can't do much even with sockets, fifos or character devices. (symbolic links are still supported since we follow them)

drop unnecessary temp variable in main_restart_track

avoid unnecessary initialization we're just gonna memcpy all over path again a couple of lines before; a smart compiler will probably ignore the initialization anyway.

simplify main_send_player: data is always NULL

don't send the song' path to the player process we're not relying anymore on the file extension, so this information is useless for the player.

reuse main_send_player instead of filling the params for imsg_compose_event.

no point in returning int from a __dead function

drop now unused #include <sndio.h> with the recent refactoring, sndio is only visible in player.c

drop functions prototypes private to player.c and move hdl there too