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In the DEBUG rts, track when CAFs are GC'd

This resurrects some old code and makes it work again.  The idea is
that we want to get an error message if we ever enter a CAF that has
been GC'd, rather than following its indirection which will likely
cause a segfault.  Without this patch, these bugs are hard to track
down in gdb, because the IND_STATIC code overwrites R1 (the pointer to
the CAF) with its indirectee before jumping into bad memory, so we've
lost the address of the CAF that got GC'd.

Some associated refactoring while I was here.
CheckUnload needs to look at revertible_caf_list

Retained CAFs must keep an object file alive.
Allow the linker to be used without retaining CAFs unconditionally

This creates a new C API:

   initLinker_ (int retain_cafs)

The old initLinker() was left as-is for backwards compatibility.  See
documentation in Linker.h.