bash-completion: journalctl query by binary and device

bash-completion: journalctl query by binary and device

The approach taken is different between the two:
- since there are many files in /usr, but messages appear
  only for a tiny subset, the completion is performed
  only for stuff shown by journalctl -F _EXE. This makes
  sense because the list is already in proper form.
- since it is hard to convert _KERNEL_DEVICE to device
  file name, simply all files in /dev/ are used as possible
  completions.

Unfortunately zsh completion requires more work and is not
covered by this commit.