terminal: make utf8 decoder return length

Lets return the parsed length in term_utf8_decode() instead of a buffer
pointer. Store the pointer in the passed argument.

This makes it adhere to the systemd coding-style, were we always avoid
returning pointers, but store them in output arguments. In this case, the
storage is not allocated, so it doesn't fit 100% to this idiom, but still
looks much nicer.