plugin.c: prevent re-adding read functions

plugin.c: prevent re-adding read functions

When multiple LoadPlugin options are used with a plugin, the plugin is
only loaded once, however the plugin's read function is invoked
multiple times at every interval. This in turn causes undesired
redundant data to be sent to the write plugins.

This patch prevents a plugin's read function from being re-added to
the read_heap and read_list during plugin registration.

Signed-off-by: Blaise Tarr <blaise.tarr@gmail.com>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>