Simplify warn.py and output of warning count table and tablerow.

* Use a loop to initialize empty 'members' and 'option' of warnpatterns.
* Create severity.{color,columnheader,header} from an attributes list.
* Compute totalbyproject and totalbyseverity from a constructor.
* Skip all-zero rows and columns in the warning count table.
* Remove redundant spaces and newlines in tablerow output.

Test: run through build.log files
Change-Id: I4d3fa4ecd92e1afab91d85b7535d03f8696e83d8