For sub-boards configured to export "ASCII Only", convert UTF-8 correctly

This converts the body text as well as the to, from, and subject header
fields (first to CP437, then to ASCII).

This should address the issue raised in issue #386 with exporting UTF-8
sequences to some message networks.

src/sbbs3/sbbsecho.c

@@ -4824,6 +4824,13 @@ void export_echomail(const char* sub_code, const nodecfg_t* nodecfg, bool rescan
 				smb_freemsgmem(&msg);
 				continue;
 			}
+			if((scfg.sub[subnum]->misc & SUB_ASCII) && smb_msg_is_utf8(&msg)) {
+				utf8_to_cp437_inplace(buf);
+				utf8_to_cp437_inplace(hdr.to), ascii_str(hdr.to);
+				utf8_to_cp437_inplace(hdr.from), ascii_str(hdr.from);
+				utf8_to_cp437_inplace(hdr.subj), ascii_str(hdr.subj);
+			}
+
 			lprintf(LOG_DEBUG,"Exporting %s message #%u from %s to %s in area: %s"
 				,scfg.sub[subnum]->code, msg.hdr.number, msg.from, msg.to, tag);
 			fmsgbuflen=strlen((char *)buf)+4096; /* over alloc for kludge lines */