sysusers: cross-check user and group names too

[lucab]

This adds an additional name check when cross-matching new group entries against existing users, which allows coalescing entries matching both ID and name.
It provides a small idempotence enhancement when creating groups in cases where matching user entries are in place.
By fine-tuning the conflict detection logic, this avoids picking up new random IDs and correctly prefers configuration values instead.

Refs:

• systemd-tmpfiles-setup.service can skip running in the real root #23745 (comment)
• systemd-sysusers reporting it is creating root group with GID 999 in initramfs coreos/fedora-coreos-tracker#1297

[bluca]

Looks fine, but could you please add a testcase covering this in test/test-sysusers.sh.in? Given it's a corner case, otherwise it will risk regressing

[poettering]

hmm, can you give an example what previously wasn't allowed and now is?

[lucab]

Rebased, const-ified, and added a new test-sysusers/test-15 entry to cover this case.

It fixes the following scenario (observed in the wild as a somehow degenerate side-effect of nss-altfiles forwarding to initrd):

$ cat etc/passwd 
root:x:0:0:root:/root:/bin/bash

$ cat etc/sysusers.d/testcase.conf 
u root    0     "Super User" /root

$ systemd-sysusers --root .
Creating group 'root' with GID 999.

[poettering]

lgtm