- libudev: return errno or set errno

 - delete mobile-action-modeswitch
   (moved to usb_modeswitch)

 - /run/udev/control socket
   (add ConditionVirtualization=!pidns)

 - kill rules_generator:
   - no longer rename netifs in kernel namespace
   - drop persistent optical names
   - do not create rules on-the-fly, require explicit configuration
     for netifs and optical drives
   - properly document what system management needs to put
     into rules files

 - kill tabs?

 - remove deprecated trigger --type=failed logic

 - remove deprecated BUS=, SYSFS{}=, ID= keys