package/systemd: register NSS plugins in nsswitch.conf

package/systemd: register NSS plugins in nsswitch.conf

This supports 4 plugins, each will be added at the right spot if
enabled, based on the template coming with systemd.

The sed replacements are carefully written to be idempotent, and to
be robust enough to be combined with the other available packages
(nss_mdns4) in any installation order.

nss-systemd is used for the DynamicUser features, which is a defacto
necessity for systemd. It handles transient users/groups without
touching the /etc/{passwd,group} files on disk. To support the
'SupplementaryGroups' feature, groups should be merged.

nss-myhostname allows resolving the hostname, again without touching
files in /etc.

nss-mymachines adds name resolution from containers supported by
machined. Users from the containers might end up in system groups, so
groups should be merged.

nss-resolve, part of resolved, is required for consistent dns lookups.
As per the documentation (nss-resolve(8)), DNS queries shall not
continue past the resolve service, unless the service is not available.

We anchor nss_resolve to appear after files, if mymachines is also used,
remove that first (and add it back later). Other packages (mdns4) move
around the dns entry, so replacing that is not a good option.

If mdns4 is installed aswell, then resolved will take precedence for
host lookups.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
[yann.morin.1998@free.fr:
  - exp[lain why 'host: resolve' uses !UNAVAIL=return
  - rewrap commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>