localed: also report when we couldn't convert X11→console

localed: also report when we couldn't convert X11→console

Rework the code a bit where find_converted_keymap cannot (and should not) be
called with a null layout, so streq can be used instead of streq_ptr, etc.

Note that the behaviour of vconsole_convert_to_x11 and x11_convert_to_vconsole
is not symmetrical. When the latter cannot find a match, it simply makes the
vconsole mapping empty. But vconsole_convert_to_x11 leaves the x11 layout
unchanged. I don't know what the proper solution is here, so I'm just adding
more verbose logging without changing the logic.