localedata: hu_HU: fix multiple sorting bugs (bug 18934)

Fix the incorrect sorting order of a digraph and its geminated variant,
regression introduced by a faulty fix to bug 13547 in commit
b008d4c8.

Fix two inconsistencies in sorting unusual capitalization of digraphs
(bug #18587).

Enable DIACRIT_FORWARD to work around bug #17750.

Sort foreign accents after the Hungarian ones.

Add extensive unittests containing all the examples from The Rules of
Hungarian Orthography and many more, including explanatory comments.