systemd/TODO

 - test (now fixed) /dev/tape/ links

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

 - move udevd -> --libexecdir

 - allow path_id, usb_id, usb_db, pci_db as built-in?

 - 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

 - move udevadm -> --bindir

 - remove deprecated trigger --type=failed logic

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

 - remove RUN+="socket:"

 - kill tabs?

 - libudev: return proper errno or set errno