Adding a -t configuration flag that will make biboumi check the
configuration file syntax.

Such a syntax check is handy for some deployment tools. It helps the
tooling to statically analyse the configuration file and prevent
unecessesary deployments when the configuration file is clearly
incorrect.

Updating the synopsis with this new flag.

Since private messages are now always coming from the server-wide JIDs

This has the following advantages:

- Works while all resources are offline (persistent channels)
- Helps with combating duplicate messages when sent to different
  resources, resources going offline, carbon-copying etc. etc.

As a side effect, this also makes the MUC PMs not be MUC PMs and
always be emitted from the server-wide JID of the sending user.

Fixes #3313.

Instead of just one. This fixes the issue of the "trailing whitespace" since we
now split it on ' '

Fix #3442

Makes the build fail, on some condition.

fix #3434

This way, this will use either the docker or the podman runners.

The other steps still use only docker, because I didn’t solve the cache
issue on podman yet.

Fix #3431