Implement enum type for guc parameters, and convert a couple of existing

Implement enum type for guc parameters, and convert a couple of existing
variables to it. More need to be converted, but I wanted to get this in
before it conflicts with too much...

Other than just centralising the text-to-int conversion for parameters,
this allows the pg_settings view to contain a list of available options
and allows an error hint to show what values are allowed.