minor changes to petsc-dev directory structure

[Barry Smith <bsmith@xxxxxxxxxxx>]

  $PETSC_ARCH/conf/petscconf -> $PETSC_ARCH/conf/petscvariables


  This is to match the conf/variables conf/rules layout. Need some  
sort of prefix  in the PETSC_ARCH
directory version so that when PETSC_ARCH is '' they are still there  
with different names then
variables and rules