Allow exporting kOS-C identifiers to other kOS scripts

[fatho]

This could consist of adding an export keyword (or the like) that always causes a function to be generated in the output file, even if it is not reachable from the main function.
Then it should also be allowed to compile files without main function as a library of some sort.

The reason for this is that it allows other people not wanting to install this compiler or to write their programs in kOS-C to still profit from libraries targeted at kOS-C.