[rfa/doco] PROBLEMS: add regressions since gdb 6.0

[Michael Elizabeth Chastain]

> There are problems in all the releases, we don't list them.

The title of the file is "Known problems in GDB 6.1".
Indeed, I would like it to actually list all the known problems.
We just don't have the resources to do this yet.

> The objective here is to draw the users attention to places where gdb is 
> both better and worse than the _last_ release (6.0).  Not a sorry list 
> of problems dateing back to version 0.0.

NEWS dates back to version 0.0, so I think that PROBLEMS should as well.

Michael C