src/plugin.c: Initialize plugins before checking if read callbacks are available.

src/plugin.c: Initialize plugins before checking if read callbacks are available.

plugin_init_all() checks if any read callbacks are available before starting
the read threads. A few plugins register their read callback from their
init callback though. By initializing the plugins before that the read
threads are still started correctly if all plugins register the read
callback in the init callback.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>