Cleanup sweep and fix a bug in RTS flag processing.

Cleanup sweep and fix a bug in RTS flag processing.

This code has accumulated a great deal of cruft over the years, this
pass cleans up a lot of the surrounding cruft but leaves the actual
argument processing alone - so there's still more that could be done.

Bug fixed:

  - ghc_rts_opts should not be subject to the --rtsopts setting.  If
    the programmer explicitly declares options with ghc_rts_opts, they
    shouldn't also have to accept command-line RTS options to make them
    work.