agent: Don't be loud about our DBus session bus address

Removes this line from logs:

cockpit-agent[802]: dbus-daemon output line: unix:abstract=/tmp/dbus-sAAn6y...

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
agent: Don't complain about files in /usr/share/cockpit

The code tries to treat everything  in /usr/share/cockpit
as a directory. If there's a file there, ignore it.

Fixes this log message:

cockpit-agent[802]: sshd-reauthorize.pam: Failed to open file
   '/usr/share/cockpit/sshd-reauthorize.pam/manifest.json': Not a directory

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
daemon: Quiet down cockpitd startup and shutdown

It's really just a tool, and doesn't need to be noisy unless
it fails.

Closes #1166
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>