Let plugin_dispatch_values() set value_list.time in case of 'now'.

Let plugin_dispatch_values() set value_list.time in case of 'now'.

Basically any (read) plugin was calling time(NULL) to set the time of a values
list. This is now done by plugin_dispatch_values() in case the time is zero
(which is the case when using VALUE_LIST_INIT or VALUE_LIST_STATIC).

This change slightly simplifies the code and makes future changes to the time
management (like the possibly upcoming subsecond resolution support) less
intrusive in respect to the amount of required code changes.