Various enhancements

The syntax of the `time` argument does not comply with what is documented in `shutdown(8)`.  Attempt to fix this.

While here, merge code for rebooting and shutting down (as it is already done in the DDL).  This will generate conflicts with #1 and #2 so in order to avoid creating a log of pull requests wich generates conflicts, I grouped everything in this branch.  Please cherry-pick the parts you are interested in.

For the record, the master branch of sante-link/mcollective-icanhazip can be used as a reference of these 3 pull requests together.