gdb/remote: remove some manual reference count handling

While working on some other target_ops reference count related code, I
spotted that in remote.c we do some manual reference count handling,
i.e. we call target_ops::incref and decref_target (which wraps
target_ops::decref).

I think it would be better to make use of gdb::ref_ptr to automate the
reference count management.

So, this commit updates scoped_mark_target_starting in two ways,
first, I use gdb::ref_ptr to handle the reference counts.  Then,
instead of using the scoped_mark_target_starting constructor and
destructor to set and reset the starting_up flag, I now use a
scoped_restore_tmpl object to set and restore the flag.

The above changes mean that the scoped_mark_target_starting destructor
can be completely removed, and the constructor body is now empty.

I've also fixed a typo in the class comment.

The only change in behaviour after this commit is that previously we
didn't care what the value of starting_up was, we just set it to true
in the constructor and false in the destructor.

Now, I assert that the flag is initially false, then set the flag true
when the scoped_mark_target_starting is created.

As the starting_up flag is initialized to false then, for the assert
to fire, we would need to recursively enter
remote_target::start_remote_1, which I don't think is something we
should be doing, so I think the new assert is an improvement.