Support sending percentages for memory utilization

Allow setting ReportPercentage and ReportAbsolute to enable
sending percentages of memory usage instead of absolute values.
This is useful when trying to compare across a heterogeneous
set of systems.

Signed-off-by: Florian Forster <octo@collectd.org>
memory plugin: Use the "complex" configuration.
memory plugin: Move calculation of the percentage to memory_submit().

This removed the need for a lot of duplicate calls. It also changes the
naming schema to use the "percent" type rather than the "memory" type,
putting the information that these are percentages into the type
instance field.
collectd.conf(5): Document the memory plugin's Values{Absolute,Percentage} options.
Merge branch 'master' into katzj/memory
memory plugin: Use the new plugin_dispatch_multivalue() interface.

This takes calculation of percentages off of our hands.
collectd.conf(5): Correct default value used.