Automatically escape dashes in interface names

Custom interface names like "eth-phone" or "eth-lte" are not uncommon,
and since they are used in pathes inside systemd units, dashes and unprintable
characters shall be escaped per unit file rules.

So replace "-" with "\x2d" in interface names.
Update documentation

DAD is still causing problems/confusion. This should help a bit.
Fix rfkill

Redirecting inside a do_debug call is, of course, impossible.
Introduce sleep hook (FS#33829)

This adds a store/restore hook to the sleep target of systemd.
Thanks to: Claudio Kozický.