Replace --foreground with --background

[WhyNotHugo]

Always run in foreground by default. Service manager and supervisors expect services to run in foreground, so they can track the lifetime of the child process. Those that don't implement their own logic to fork daemons anyway.

For users running usbmuxd manually, running in background by default provides a confusing experience, since it gives the impression that usbmuxd has exited immediately.

Anyone needing to run usbmuxd in background can use the --background option.

[WhyNotHugo]

Gentle bump