src/plugin.c: Make plugin_load() idempotent, i.e. don't load plugins twice.

Previously, multiple 'LoadPlugin "foo"' lines would dlopen the same .so file
twice and call its module_register function twice. This would eventually result
in complaints about callbacks with the same name being registered. With this
patch each plugin is only loaded only once, allowing people to write "self
contained" configuration segments.

Thanks to Tim Bunce for suggesting this!

Github: #333
collectd.conf(5): Document the new "LoadPlugin" behavior.

Github: #333
src/plugin.c: Move the perl and python "global" magic to plugin_load().

The config file handling is really not the best place to have this. Also,
the feature I'm working on would have to copy this magic if left in the
config code, and I *really* don't want that.
src/plugin.c: Fix a comment regarding duplicate plugin_load() calls.
src/configfile.c: Add the "AutoLoadPlugin" option.

When enabled, <Plugin ...> blocks will automatically load plugins.
Thanks to Tim Bunce for suggesting this!

Github: #333