shared: utf8 - support ucs4 -> utf8

Originally we only supported ucs2, so move the ucs4 version from libsystemd-terminal to shared
and use that everywhere.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
shared: utf8 - support decoding the full utf16

We originally only supported the BMP (i.e., we treated UTF-16 as UCS-2).

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>