This PR implements the first part of RFE #8046. I.e. this allows to
write:
```
u username -:300
```
Where the uid is chosen automatically but the gid is fixed.
On Debian/Ubuntu systems the default passwd/group files use a
slightly strange mapping. E.g. in passwd:
```
man:x:6:12::/var/cache/man:/sbin/nologin
```
and in group:
```
disk:x:6:
man:x:12:
```
This is not supported in systemd-sysusers right now because
sysusers will not re-use an existing uid/gid in its normal
mode of operation. Unfortunately this reuse is needed to
replicate the default Debian/Ubuntu users/groups.
This commit enforces reuse when the "uid:gid" syntax is used
to fix this.
I also added a test that replicates the Debian base-passwd
passwd/group file to ensure things are ok.
The DHCP code in systemd-networkd relies on the
`net.ipv4.conf.{default,all,<if>}.promote_secondaries` sysctl to be set
(the kernels default is that it is unset). If this sysctl is not set
DHCP will work most of the time, however when the IP address changes
between leases then the system will loose its IP.
Because some distributions decided to not ship these defaults (Debian
is an example and via downstream Ubuntu) networkd by default will now
enable this sysctl opton automatically.
Michael Vogt