Simplify the interface section

Use explicit imports. The default C function name is the lower case Fortran
name (no need to use "name="). Use just one interface block for all functions.

You don't have to use this PR. I just wanted to show how I write these things, e.g. I prefer explicit imports (the `only` keyword). I usually use the `import` statement in interface blocks these days as in this PR, but your original version might arguably be a bit simpler.