Fix #40, #39: initctl plugin started too early, before /run FS existed

Delay setup of initctl plugin to after the base filesystems are up and
after the bootmisc plugin has run.

This patch factors out the I/O initialization of plugins so it does not
have to run when plugins are loaded.  Instead, plugins can now call this
API to register an I/O callback when they are ready.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugin.h

@@ -130,6 +130,8 @@ typedef struct plugin {
 } plugin_t;
 
 /* Public plugin API */
+int plugin_io_init    (plugin_t *plugin);
+
 int plugin_register   (plugin_t *plugin);
 int plugin_unregister (plugin_t *plugin);