Fortran: PCMGResidual_Default needs to handle double-underscore

When PETSC_HAVE_FORTRAN_UNDERSCORE_UNDERSCORE, we must append two
underscores.  If this function is to be public (as it seems to be
according to the man page), it should be named PCMGResidualDefault.
Merge branch 'jed/pcmg-residual-underscore' (early part) into maint

* 'jed/pcmg-residual-underscore' (early part):
  Fortran: PCMGResidual_Default needs to handle double-underscore