organisation of C exports

[jangorecki]

I think our C exports should be better organized.

• separate C file
• exported functions should be only wrappers
• exported functions should have prefix DT_

This way will be easier to maintain our C exports.
We should then update manual and NEWS entry about the export added by @lsilvest in #4084.
There are two other requests for C exports: forder #4015 and alloccol/setDT #4439
I hope we can put them together in next release.

[jangorecki]

All points listed in this issue has been addressed in #4645
I will close it by extending documentation