input_plugin: added methods init(), finish()

Instead of hard-coding the plugin global initialization in
input_stream_global_init(), make it walk the plugin list and
initialize all plugins.

src/input/curl_input_plugin.h

@@ -21,8 +21,4 @@
 
 extern const struct input_plugin input_plugin_curl;
 
-void input_curl_global_init(void);
-
-void input_curl_global_finish(void);
-
 #endif