Complete support for UTF8 encoding conversion

Complete support for UTF8 encoding conversion

All the conversions already always go through the MBS form so
simply always convert to MBS before trying to convert to any other encoding
in the `archive_mstring_get_*` functions.

* Do this by calling `archive_mstring_get_mbs` to avoid duplicating code
* Add `struct archive*` parameter to `archive_mstring_get_mbs_l`
* Add test for set/get/update utf8 functions on entry
* Add test for encoding conversion.

Fix #931