Use the new cur_instance variable to get the call handle instead of calling
vpi_handle(vpiSysTfCall, 0).
This completes a proper fix for issue #141, to replace the problematic fix
that was reverted in commit 8da8261f.
The code to handle real functions using the PLI 1 interface was
missing some functionality. This patch fixes that and fixes a
bug in the veriuser.h include file.