Fix formatting of multi-bit flags with partial overlap

[KodrAus]

Fixes #315

cc @mxk

I've added a repro for the bug reported here: #310 (comment)

It looks like we'll need to tweak the iter_names method to make sure we include any remaining bits when they overlap, but don't complete, a multi-bit flag.

[KodrAus]

Ok, it looks like the fix for this was fairly straightfoward: we were using the flags themselves to try mask out any bits that don't correspond to a valid flag, but in this example the extra bits do correspond to part of a flag. So we'd mask them out without printing anything for them.

Using the iterator as the source of these bits ensures anything not printed by name will get printed as a hex number.

[mxk]

Thanks for getting this fixed!