processes plugin: Fix counters initialization / spikes

processes plugin: Fix counters initialization / spikes

processes plugin generates spikes on (re)start. That is caused by
wrong counter logic: When collectd (re)started, monitored processes,
which was started before collectd, have non-zero values in
cpu_user/cpu_system/vmem_minflt/vmem_majflt, so `want_init` is false.
What produces spike.
At other hand, processes which are started between read cycles, should
be fully accounted without initialization. So, we must skip only first
cycle for these counters.