udbsync: update both nfsroot_staging and nfsroot_ro in udbsync.passwd

[zopieux]

Fixes #169.

Tested:

$ machinectl shell myrhfs0
$ diff -u /export/nfsroot_{staging,ro}/etc/group
  ( empty )

$ systemd-nspawn -D /export/nfsroot_staging
  $ pacman -S minidlna
  ...
  Creating user minidlna (minidlna server) with uid 971 and gid 971.

$ diff -u /export/nfsroot_{staging,ro}/etc/group
--- /export/nfsroot_staging/etc/group   2020-03-15 21:36:38.539844642 +0100
+++ /export/nfsroot_ro/etc/group        2020-03-15 21:34:10.167058855 +0100
@@ -76,2 +76,1 @@
-minidlna:x:971:

$ systemctl restart udbsync_passwd_nfsroot

$ diff -u /export/nfsroot_{staging,ro}/etc/group
  ( empty )

[seirl]

This looks like something that could be achieved with a .path, like we do here:

https://github.com/prologin/sadm/blob/master/etc/systemd/system/rootssh.path

https://github.com/prologin/sadm/blob/master/etc/systemd/system/rootssh-copy.service

This service uses inotify to watch for changes in rootssh, and copy the changes in the appropriate directory. I think this would be a lot less hacky than having this --exec-after command.

[seirl]

No longer needed, we don't use _staging and _ro anymore.