This is to add a new category of gdb commands, "experimental".
These will be explicitly "use at your own risk", and gdb will
tell you so.

This is for new stuff under development, for example the
restore-core-file command, which are meant only for use
by savvy folks who won't be surprised if stuff goes wrong.

They're not really 'maintainance', and they're not really
'obscure', and having a separate class makes it possible
for gdb to explicitly be a nudge about them.

If and when such a command is judged mature and stable,
it will be moved into one of the traditional classes.