Support starting without any devices and auto discovering them
Even when ordered after systemd-logind.service, starting Weston from a systemd service unit, it seems to be, that devices under /dev/, like /dev/dri/card0 are not ready yet.
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[17:19:12.883] logind: session control granted
[17:19:14.881] using /dev/dri/card0
[17:19:14.881] DRM: supports universal planes
[17:19:14.881] DRM: does not support atomic modesetting
[17:19:14.881] DRM: supports picture aspect ratio
[17:19:15.290] event2 - failed to create input device '/dev/input/event2'
[17:19:15.307] event1 - failed to create input device '/dev/input/event1'
[17:19:15.323] event3 - failed to create input device '/dev/input/event3'
[17:19:15.354] event4 - failed to create input device '/dev/input/event4'
[17:19:15.386] event10 - failed to create input device '/dev/input/event10'
[17:19:15.415] event11 - failed to create input device '/dev/input/event11'
[17:19:15.451] event12 - failed to create input device '/dev/input/event12'
[17:19:15.483] event5 - failed to create input device '/dev/input/event5'
[17:19:15.506] event6 - failed to create input device '/dev/input/event6'
[17:19:15.539] event7 - failed to create input device '/dev/input/event7'
[17:19:15.571] event8 - failed to create input device '/dev/input/event8'
[17:19:15.603] event9 - failed to create input device '/dev/input/event9'
[17:19:15.634] event14 - failed to create input device '/dev/input/event14'
[17:19:15.663] event0 - failed to create input device '/dev/input/event0'
[17:19:15.683] event13 - failed to create input device '/dev/input/event13'
[17:19:15.683] warning: no input devices found, but none required as per configuration.
[…]
[17:19:15.887] set mode failed: Permission denied
[17:19:15.887] Couldn't apply state for output VGA-1
[repaint] flushed pending_state 0x113f7d0
[…]