Lua plugin: turn the collectd functions into a lib

Lua plugin: turn the collectd functions into a lib

Now the module shows up in 'package.loaded':

for name, func in pairs(package.loaded['collectd']) do
    print(name, func)
end

returns:
dispatch_values	function: 0x7f38b8a472c0
log_debug	function: 0x7f38b8a47130
register_write	function: 0x7f38b8a47520
log_info	function: 0x7f38b8a470d0
register_read	function: 0x7f38b8a47360
log_warning	function: 0x7f38b8a47070
log_error	function: 0x7f38b8a47100
log_notice	function: 0x7f38b8a470a0