mdns: die iff the MDNS advertising process dies

and not if any other child does!
provide -B/-E for running shell commands on begin/end of playing

Implements functionality described by MaZderMind <github@mazdermind.de>
in issue #201.
LICENSES updated w/ alac.c pointer
add a pipe/FIFO audio backend
die/warn functions now have implicit newline at the end
TODO updated