This deprecates --vmodule (not available in absl, mapping it to --v for
a very minimal sort of compatibility) and adds the absl::log flags
--minloglevel, --stderrthreshold, --log_backtrace_at, and --log_prefix.
This feeds into efforts to create a working install target.
The order of headers is still funky, and was "fixed" by clang-format,
but in a way that doesn't exactly align with the style guide. Further
cleanup of header order is coming in a follow-up PR.
Joey Parrish
KongQun Yang