Switch to PETSc naming for solver options

We already use PETSc options names in Firedrake, and since it's the only
linear algebra backend we support just use the PETSc names
directly. This way, it's much easier to adjust to PETSc option name
changes (or new options appearing), they're already supported.
Merge pull request #236 from OP2/rationalise-solver-parameters

Since we only support PETSc as a linear algebra backend, just use their
options naming.  Translate to CUSP names in the cuda backend for now.