sysuser: use OrderedHashmap

sysuser: use OrderedHashmap

This means we have more predicable behavior for "u foo uid:gid" lines
and also makes the generated files appear in the same order as the
inputs. So e.g.
```
u      root         0 -     /root
u      daemon       1 -     /usr/sbin
u      games        5:60 -     /usr/games
```
will generate
```
root:x:0:0::/root:/bin/sh
daemon:x:1:1::/usr/sbin:/sbin/nologin
games:x:5:60::/usr/games:/sbin/nologin
```