localed: check for partially matching converted keymaps

localed: check for partially matching converted keymaps

If a user specifies multiple X11 keymaps, with a (at least the first
one) nonempty variant, and we don't match the whole combo, use
a converted keymap which includes the variant in preference to
the default, variantless, keymap.

E.g.: We would convert X11 config "layout=fr variant=mac" to "fr-mac",
but "layout=fr,us variant=mac," to "fr", because we don't have a
converted keymap which would match "fr,us", and we don't have a legacy
mapping for "fr,us". This is unexpected, and if we cannot match both,
it is still better to match the primary mapping and use "fr-mac".