Logging: Honor G_MESSAGES_DEBUG also for journal logging.

This allows the use of structured logging and debug output at the same
time.
Tests: Log debug messages.

* Cockpitd and cockpit-ws run with G_MESSAGES_DEBUG=all.
* Only journal messages of level 5 or more urgent are checked.
Logging: Give LOG_ERR priority to assertion failure messages.

By explicitly redirecting g_printerr to the journal.
Tests: Avoid "unbound variable" error in make-rpms.
Tests: Read stdout until EOF in testvm.execute.

Fixes #124.
Tests: Don't rely on trusted journal fields.
Tests: Allow losing the race against D-Bus during shutdown.
Tests: Wait for service to be listed before clicking on it.
Tests: Correctly handle return value of reload.

It's a status string, not a boolean.
Tests: Allow connection reset error during reload.