Document this convention.

Add a script to post-process .gcov files in order to stop nagging us
about excluded lines.

Teach cov-diff to handle these post-processed files.

Closes ticket 16792

Closes #17839.

Not good markdown, mind you.

Document use of coverity, clang static analyzer, and clang dynamic
undefined behavior and address sanitizers in doc/HACKING.

Add clang dynamic sanitizer blacklist in
contrib/clang/sanitizer_blacklist.txt to exempt known undefined
behavior. Include detailed usage instructions in this blacklist file.

Patch by "teor".

cypherpunks authored 10 years ago and Nick Mathewson committed 10 years ago

Additional fixes to make the change work;
- fix Python 2 vs 3 issues
- fix some PEP 8 warnings
- handle paths with numbers correctly
- mention the make rule in doc/HACKING.