[unwind-20030108-branch] Add sentinel-frame + misc

[Andrew Cagney]

> It's hardly a bug; more of a deep interface shortcoming.  Consider:

Which is a bug.

> File A declares a type 'struct foo'.
> File B declares a type 'struct foo'.
> Header C forward declares 'struct foo;' and 'struct foo *get_foo ()'.
> File D includes Header C and uses 'get_foo'.
> 
> I'm sitting in File D, and I want to look in the debugger at this
> opaque unwind cache.  There's no possible way to associate it with
> either struct foo.
> 
> That's why I think the construct should be avoided; if one of them is
> A_foo and the other is B_foo, and we do a bit of casting, then at least
> I can dump the one I want.

Shouldn't something like:
	('File A:struct foo' *) get_foo()
work?  Even:
	(gdb) list 'File A'
	(gdb) print *(struct foo) get_foo()
How does GDB differentiate between variables and enum's when they have 
multiple declarations?

Andrew