Merge branch 'ds/scalar-no-maintenance'

Two "scalar" subcommands that adds a repository that hasn't been
under "scalar"'s control are taught an option not to enable the
scheduled maintenance on it.

* ds/scalar-no-maintenance:
  scalar reconfigure: improve --maintenance docs
  scalar reconfigure: add --maintenance=<mode> option
  scalar clone: add --no-maintenance option
  scalar register: add --no-maintenance option
  scalar: customize register_dir()'s behavior