Delete generated files, clean up Debian/SteamOS packaging

[smcv]

• debian: Remove generated files by running dh_clean
  • This removes built files that were accidentally included in the .dsc that we imported (presumably someone used dpkg-source -b directly, instead of dpkg-buildpackage).
• debian/.gitignore: Add
  • This avoids the same built files getting included in the source package again.
• Remove generated files
  • Similar to the above, but in the "upstream" source code.
• .gitignore: Add
  • Similar to the above, but in the "upstream" source code.
• debian: Wrap (build-)dependencies one per line
  • This just makes diffs easier to read.
• debian: Use default version of automake
  • Automake 1.11 was ancient even in SteamOS 1.
• debian: Use debhelper 10 compat level
  • This uses dh-autoreconf by default, which makes sure we regenerate the Autotools-generated files every time.
• debian: Don't pass discouraged --ignore-missing-info option to dpkg-shlibdeps
  • Needing this option nearly always indicates a bug.
• debian: Remove unnecessary explicit dependencies.
  • ${shlibs:Depends} generates these automatically, and if it doesn't, there is a bad bug somewhere.