Code health

[alalamav]

• Remove/refactor any extraneous/legacy cruft (e.g. Psiphon-specific stuff?)
• Refactor any spaghetti, non-DRY, or error-prone code
• Grab any config-type values (e.g. DNS resolver address) from config rather than hard-coding
• Adopt suggestions from https://github.com/akullpp/awesome-java and https://github.com/aleksandar-todorovic/awesome-c that would be helpful
• Unit, functional, fuzz, and/or property-based tests
• Continuous Integration (+ status badge link in README if possible). Make sure all of the following run successfully:
  • Linter
  • Automated tools to catch memory-safety issues and other issues not caught be the compiler
  • Building the code for all important targets
  • Automated tests
• Code coverage (+ status badge link in README if possible)
• Javadoc comments for public interfaces
  • Generate html documentation (+ status badge link in README if possible)
• Standardize return 0|1 statements
  • return ERRCODE|SUCCESSCODE (ref: DNS over UDP relay #13 (comment))

[jab]

Migrated much of this to https://buganizer.corp.google.com/issues/37267129, but a few things are specific to this codebase. So leaving this open here too, until those are migrated to Buganizer or fixed on master.