[plugins] Modification of most of the plugins.

+ net/active
+ net/bruteforce
+ web/semi-passive
+ web/active
+ web/external
[plugin] Merge AbstractRunCommandPlugin into ActivePlugin.
[plugin] Add ATTR attributes and fix typos.
[web/active] Fix typos.
[plugin] Homogenize class names.
[plugin] Update plugin manager and handler.

+ plugin_manager retrieves the information about a plugin with the classful
system.

+ plugin_handler creates an instance of the plugin class and run it with the
classful system.