Creates systemd-resolved symbolic link to /etc/resolv.conf

NetworkManager (etcnet) is default now.

NM_CONTROLLED should always be set together with DISABLED;
otherwise, it's possible to reach the state where
DISABLED=no but NM_CONTROLLED=yes, and have the interface
controlled by both NM and etcnet.

When renaming HOSTNAME in the alterator, switches managing interface to etcnet.
This creates problems for users.
This partial reverts commit d4170557.

This reverts commit 226a2395.

Users are getting upset about NetworkManager being unable to
configure "System EthX" connections that are meant to set up
with "acc" (which isn't obvious at all).

See-also: https://forum.altlinux.org/index.php?topic=42519.0

Putting any configuration into /etc/net/ifaces/lo/resolv.conf
makes etcnet *overwrite* /etc/resolv.conf, while putting that
into /etc/resolv.conf itself makes e.g. vzctl --nameserver
*append* to what's been specified.
Reported-by: Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>

Refer to net-dns feature where appropriate
(it actually started out as an extension of
net feature but the reasons to separate it
quickly became apparent to me).

Using just a single one of those tends to end up problematic
at times but any two use to suffice.

This needs further refinement regarding p7/t7 specifically:
NM behaviour regarding defaults differs in sisyphus and this
has led to livecds booting with DHCP networking but installed
systems booting without configured interfaces.

50-setup-network was a hasty hack (surprise!) that used to do
what net and net-eth features have been created to do since;
just drop the duplicated crufty code.

Unconditional resolver setup isn't done now: those with static
setup are better off doing it explicitly, and those with DHCP
should be fine already.

NB: /etc/hosts *is* fine within setup package *but* hasher will
    overwrite it with a copy of host's one; let's reset contents
    to initial at least until hasher gets fixed and the fix is
    rather deployed in the wild.