Refactor flooddetach to use command callbacks

This gives a proper help command "for free", and makes
it more pleasant to add new commands (silent) later.
flooddetach: make secs & lines commands rw

Improves the usability of the module.
flooddetach: add command "silent [yes|no]"

Inspired by the route_replis module. Resolves #591.
Merge pull request #633 from jpnurmi/flooddetach

flooddetach: add command "silent [yes|no]"