Upgrade initialization code in vms/vms.c.

Upgrade initialization code in vms/vms.c.

The LIB$INITIALIZE program section is examined by the image
activator for an array of 32-bit pointers to functions that will
get called early in start-up, before main() is called.  We use
this mechanism to get and set various run-time features.

The implementation we had was a bit cluttered with undocumented
features that weren't being used, and it didn't work under C++.
This new implementation is simpler and follows the documented
usage in the C++ release notes much more closely, and it works
under both C and C++.

We also now explicitly export the LIB$INITIALIZE psect in the
linker options file used to create the PERLSHR shareable image.