With the recent changes to the script it does not incur a startup delay
by default due to starting youtube-dl and waiting for it. This was the
main reason for making libmpv have a different default.
Starting sub processes from a library can still be a bit fishy, but I
think it's ok. Still mention it in the libmpv header. There were already
other cases where libmpv would start its own processes, such as the X11
backend calling xdg-screensaver. (The reason why this is fishy is
because UNIX process management sucks: SIGCHLD and the wait() syscall
make sub processes non-transparent and could potentially introduce
conflicts with code trying to use them.)