Let avahi handle mdns requests

We need to disable it by default in resolved so that it doesn't fight
with avahi for the port when both are started up in parallel.

I also moved nss-files before nss-resolve. This is unfortunate because
resolved cached files and with the move, the file will be re-read on each
query. Nevertheless, we want nss-files to have higher priority than nss-mdns
to honour local config. Fortunately, only some people put lots of entries
in /etc/hosts, so the inefficiency incurred by this isn't important for
most users.

nss-myhostname is moved after nss-files, following the change in
upstream recommendations.

systemd.spec

@@ -385,7 +385,8 @@ CONFIGURE_OPTS=(
         -Dversion-tag=v%{version}-%{release}
         -Dfallback-hostname=fedora
         -Ddefault-dnssec=no
-        -Ddefault-mdns=resolve
+        # https://bugzilla.redhat.com/show_bug.cgi?id=1867830
+        -Ddefault-mdns=no
         -Ddefault-llmnr=resolve
 )
 
@@ -660,7 +661,7 @@ function mod_nss() {
         # Add nss-resolve to hosts
         grep -E -q '^hosts:.* resolve' "$1" ||
         sed -i.bak -r -e '
-                s/^(hosts):(.*) files( mdns4_minimal .NOTFOUND=return.)? dns myhostname/\1:\2 resolve [!UNAVAIL=return] myhostname files\3 dns/
+                s/^(hosts):(.*) files( mdns4_minimal .NOTFOUND=return.)? dns myhostname/\1:\2 files\3 resolve [!UNAVAIL=return] myhostname dns/
                 ' "$1" &>/dev/null || :
     fi
 }
@@ -801,6 +802,8 @@ fi
 %changelog
 * Wed Sep  2 2020 Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> - 246.4-1
 - Create /etc/resolv.conf symlink upon installation (#1873856)
+- Move nss-mdns before nss-resolve in /etc/nsswitch.conf and disable
+  mdns by default in systemd-resolved (#1867830)
 
 * Wed Aug 26 2020 Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> - 246.3-1
 - Update to bugfix version (some networkd fixes, minor documentation