And make a function for the 'rescan' subcommand so that we are
consistent and have only subcommand logic in the main function,
which return command_still_pending() in any case (but 'stop').
patched-by: @rustyrussell
This does the same as for the 'start' subcommand for the 'startdir'
one.
Note that we could fail to start the last plugin of a directory, but
have succesfully started the precedent plugins. This will make us return
an error to the user while some of the plugins have been started, but we
still don't end up in a transient state with
half-configured-half-errored plugins.
This remove the reliance on startup plugins' function "plugin_start" in
order to use a distinct runtime path for a dynamically started plugin,
which will allow startup plugins' code to be (almost) agnostic of
dynamic plugins.
This also makes the 'start' subcommand return only if the plugin is
either started, or killed : no weird middle state where the plugin
mishbehaving could crash lightningd.
This adds a new pair of files : lightningd/plugin_control, along with a new RPC
command : 'plugin'. This command can be used to manage plugins without restarting lightningd:
lightning-cli plugin start helloworld.py
lightning-cli plugin stop helloworld.py
