(bug 47216) API: indicate default and skipped skins in meta=siteinfo

action=query&meta=siteinfo&siprop=skins can easily indicate which of the
returned skins is the default and which are unusable (e.g. because
they're listed in $wgSkipSkins). So we may as well do so.

Bug: 47216
Change-Id: Ib4ea5fe85e1b02895dba15f3a245c7a7d8724470