systemd/TODO

 - remove RUN+="socket:"

 - use 'udevd:' in logs

 - udev_enumerate_add_match_subsystem(enum, "input");
   udev_enumerate_add_match_sysname(enum, "event*");
   udev_enumerate_add_match_children(input_dev);
   udev_enumerate_scan_devices(enum);

 - monitor: use clock MONOTONIC

 - libudev: return proper errno or set errno

 - move ctrl.c from libudev/ to udev/

 - move:
     udevd -> --libexecdir
     udevadm -> --bindir

 - /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