Fix shutdown action name.

(Inconsistent with the DDL)
Fix action description.
Enforce valid time format.

According to shutdown(8), time must be one of:
  - An absolute time in the format hh:mm, in which hh is the hour (1 or 2
    digits) and mm is the minute of the hour (in two digits);
  - A relative time in the format +m, in which m is the number of minutes to
    wait;
  - The word now.
Merge reboot and poweroff actions.
Merge branch 'refactor'