imgBSD/Files: Boot and sysctl fixups

Too much was being altered by default regarding sysctl and boot
configurations of hardware. This commit reduces the volume of settings
that alter non-default values.

These changes were decided after tracking down a crackling audio issue
that may have been due to setting:

hw.snd.latency="7"

But the fact that altering this value fixed the issue could have been a
coincidence.