- Fix a couple of bugs in yesterday's pipeline cleanup.

- Do some more tidying: share the code for filename generation between
  the two entry points to HscMain, and also share some of the other
  machinery in the Hsc phase.  This fixes some wibbles (things that
  were done in --make mode but not in one-shot, and vice-versa).

  One thing that works now is that if you say 'ghc -keep-hc-files Foo.hs',
  then it automatically switches to -fvia-C mode to generate the .hc file.