Give formal index a proper title.
Use `Pass::formatted_help()` to assign the group, but still return `false` because the help text still comes from `Pass::help()`.
Tidy up some of the affected files' includes to make use of the shared `yosys.h` includes.
o Not all derived methods were marked 'override', but it is a great
feature of C++11 that we should make use of.
o While at it: touched header files got a -*- c++ -*- for emacs to
provide support for that language.
o use YS_OVERRIDE for all override keywords (though we should probably
use the plain keyword going forward now that C++11 is established)
