Use keyword.control.directive on omp-directives and clauses

[eirik-kjonstad]

Some suggested changes to omp scopes.

Adjusts scope of directives (omp, parallel, do,...) to keyword.control.directive.fortran and similarly for omp clauses (default, shared, private,...) - as suggested in #4. I kept support.constant and support.function for possible schedule values (guided, static, ...) and omp intrinsic functions (omp_get_num_threads and so on).