typefinding: detect MSS manifests without using g_convert()

typefinding: detect MSS manifests without using g_convert()

Embedded systems often have limited charset conversion
functionality, so don't rely on g_convert() (i.e. iconv)
for UTF-16 to UTF-8 conversions, we can easily enough do
that ourselves by converting to native endianness and
then using GLib's helper functions.