Take the configured origin line chars into account when determining charset

and if the message is UTF-8, encode the origin line as UTF-8 too.
If the message is straight ASCII (e.g. the sub-board is configured as ASCII
only), convert the configured origin line to ASCII, before appending.

At this time, only ASCII and CP437 encodings of originlines are supported
in Synchronet (imported and exported origin lines can contain UTF-8).

This should address the issue raise by Sergey Dorofeev (2:5020/12000) in the
FidoNet UTF-8 echo whereby messages exported from Synchronet systems
(presumably using SBBSecho) could contain a mixture of CP437 and UTF-8
encodings (!) when the sysop had CP437 chars in their configured origin line.