GDB `cannotfix' pr state, require PR with xfail `moving forward'.

[Andrew Cagney]

With revisions:

> There is currently a long thread (Remove all setup_xfail...)'s on gdb-patches@.  Several proposals, I think, can already be identified at this point in the discussion.
> 
> - yank the existing xfail PR markings (but not the actual xfails) (they apply to old internal Red Hat and HP bug databases and hence are meaningless).

Still stands.

> - `moving forward' all new xfails, and all modifications to existing xfail's should include a bug report  (this way, new analyzed vs old unanalized xfail's can easily be differentiated).

Still stands.

> - GDB have a new closed state `cannotfix'; or a new class `xfail' or `notabug' or ...; or even reuse the class `mistaken' that can be used to categorize xfail bug reports (not sure which is better here).

GDB has a new category `external'.  External bugs can either be 
`suspended' (I guess that implies that the bug is waiting on the 
external code to be fixed), or `closed' the external problem has been fixed.

Andrew