Use exclusive lock under /dev/, silence expected errors in installer

Use exclusive lock under /dev/, silence expected errors in installer

resolvd(8), slaacd(8) and dhcpleased(8) are different from other daemons
in that there must only be a single instance.

resolvd already does this, adjust slaacd and dhcpleased accordingly while
moving the lockfile paths under /dev/ such that they work early on boot and
don't run into races should /var be (un)mounted between daemon starts.

Locking is especially required in the installer where all three daemons are
started every time the "(I)nstall, (U)pgrade, (A)utoinstall or (S)hell? "
prompt is entered, i.e. restarting installation or dropping into a shell
and back into the prompt again would start multiple instances.

To avoid expected lockfile error messages in between installer prompts,
discard standard error when starting the autoconf daemons;  none of them
has other potential failure cases in installer mode before daemon(3)izing.

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