Clean field names

Non-clean field names cause problems with drawing, e.g. when there are spaces
or dashes in the process name. This commit cleans the field name by using the
appropriate support module. For more information, see:

http://munin-monitoring.org/wiki/notes_on_datasource_names
Added documentation for 'Clean field names'
Clean procargs and procuser as well

Github user @leeclemens reported that procargs should be cleaned as well and
rightly so. I took this opportunity to clean procuser as well, since usernames
could have periods (even though people should probably avoid that, if possible).

Tested procargs with 'python example.py', this gave expected results.