New XS keyword: EXPORT_XSUB_SYMBOLS

New XS keyword: EXPORT_XSUB_SYMBOLS

This can be used to enable/disable the exporting of XSUB symbols.
Technically and at least as far as the documentation change is
concerned, this change goes together with the upcoming change
to XSUB.h to make not exporting the XSUB symbols the default.
It's harmless if a bit useless without that.

In the original plan for making XSUBs "static", there wouldn't have
been a public and easy way to make XSUBs exported from XS code
without resorting to manually redefining macros. But since even in
just the core set of modules, simple things break, I think it's best
to expose this bit of choice.