[Fortran] Check for ACTION specifier in OPEN statements

The `OPEN` statement allows the `ACTION` specifier which can be `READ`, `WRITE`, OR `READWRITE`, analogous to `INTENT(IN)`, `INTENT(OUT)`, and `INTENT(INOUT)` respectively. The programmer knows how an opened file is to be used, and using a file connection in a way contrary to the `ACTION` will produce a runtime error. Adding a check that each `OPEN` includes an `ACTION` could therefore help reveal bugs where a file that is intended to be read-only is written to. This would improve code correctness and security.

It would be even better not to rely on the runtime error, and instead check that no `WRITE` statement is applied to a unit opened as `ACTION='READ'` and vice versa.