core: Changed internal API to allow for per-plugin intervals.

core: Changed internal API to allow for per-plugin intervals.

This is the basic support for configuring custom intervals on a per-plugin
basis. The following API changes have been introduced:

 - Each plugin's 'module_register()' function is now required to accept and
   respect an argument passing the custom interval (if configured). This is
   done by passing a pointer to a struct 'plugin_loaddata_t' which (currently)
   contains a single cdtime_t member storing the custom interval (0 if none
   has been configured).
 - A new plugin-global variable, 'plugin_interval', has been introduced. It is
   defined in 'plugin.h', and, thus, defined and available in all plugins.
 - A new macro, 'PLUGIN_INIT_INTERVAL', has been introduced to initialize
   'plugin_interval' in 'module_register'. It requires the passed loaddata
   object as argument. In order for this mechanism to work, this macro has to
   be used in all plugins.
 - The macro 'VALUE_LIST_INIT' now requires the default interval as an
   argument. In most cases 'plugin_interval' should be used.
 - 'plugin_register_read' now requires a third argument specifying the
   interval. Usually, 'plugin_interval' should be passed.
 - The <LoadPlugin> block now accepts an option 'Interval' to configure the
   custom interval.