Add unlink from driver to deassign and zero link pointer.
Add unlink from driver to deassign and zero link pointer.

This patch fixes deassign to allow it to unlink from a driver.
It also zeros the cassign_link and force_link pointers after
they have been unlinked. Not doing this will cause an assert
if deassign/release are called multiple times (variable only).

CastMi/iverilog,CastMi/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog