plugins: Add config_init for custom configuration

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
dbus: Move capabilities parsing to config_init

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
dbus: More static functions

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
dbus: Split directory loading to its own function

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
dbus: Pre-test directory existence

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>