Fuzzing improvements

[arcz]

This commit consists of fuzzing improvements as part of Trail of Bits audit that we want to contribute.

Summary of changes:

• harnesses moved from oss-fuzz to this repository
• fixed the harnesses so they build with the current code
• expanded coverage of the existing harnesses
• added two new harnesses: fuzz_mss and fuzz_parse_argv
• changed OpenVPN code to use the FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION macro instead of ad-hoc patching as recommended by the libFuzzer documentation
• added a simple python script to run fuzzing straight from OpenVPN repo and to generate coverage reports

After integrating those changes we recommend doing the following next steps:

• integrate building harnesses into CI to ensure they don't break with OpenVPN code changes
• update the OpenVPN project in oss-fuzz to use the harnesses from OpenVPN repository