Provide more accurate lock error message

Provide more accurate lock error message

When started manuall from single-user mode (/ still read-only), the current
error is misleading:
	# slaacd -dv
	slaacd: already running

The lock was specifically added to prevent multiple instances in the
installer, which discards the error message entirely anyway.

Retain the useful EAGAIN/"already running" message, but otherwise print the
real error reason:
	# slaacd -dv
	slaacd: /dev/slaacd.lock: Read-only file system

Feedback OK deraadt millert