Build: Fix coverage targets.

• COVERAGE_DIR should be an absolute path so it doesn't get accidentally created outside of the source/build tree.

• Use long command line arguments to everything to make it a little more clear what these commands do. We so rarely use any of these tools that no one remembers the short versions.

• Ignore unused excludes. At some point, an unused exclude became an error which stops the build process. Ignore those errors instead of trying to figure out what is and isn't used.

• Restrict code coverage to the lib/ directory in the main target. Previously, we were scanning everything and then going through and removing certain directories. Instead, let's just scan the one thing we do care about. This loses coverage checking of include/, but I'm okay with that.

• Add && before the genhtml commands. This was previously broken - the line continuation made it part of the preceeding lcov command.