journalctl: show coredumps again when --unit= is specified

journalctl: show coredumps again when --unit= is specified

This effectively reverts fdcd37df3b97abc381c7b7a29b81cc013c7a3230.

After 888e378da2dbf4520e68a9d7e59712a3cd5a830f, coredumps for system
services are stored by systemd-coredump user. Henve, _UID=0 does not
match anything anymore.

Before:
```
$ build/journalctl -u systemd-networkd.service -p2 -n1
-- No entries --
```

After:
```
$ build/journalctl -u systemd-networkd.service -p2 -n1
Dec 06 05:39:55 H systemd-coredump[4187]: [🡕] Process 4145 (systemd-network) of user 192 dumped core.
(snip)
```