sysusers: cross-check user and group names too

sysusers: cross-check user and group names too

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.