inotify: network: improve "allow-hotplug" & "auto" interaction

inotify: network: improve "allow-hotplug" & "auto" interaction

commit c86cfb8bbd9b505d06b580582297fa670561437b dropped allow-hotplug
from the primary interfaces file completely on write, but that breaks
setups that come from plain Debian.

Instead, as stop-gap measurement, transform "allow-hotplug" to auto
in the PVE controlled config.

That avoids conflict and improves installing PVE on top of plain
Debian, as the interface still comes up after the first reboot.

But it is not ideal auto is not the same as hotplug, so we need to
also track that difference in the future, but that needs some
adaptions in the API too (change autostart from boolean to
string+enum or so=

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>