If a message header has a thread_back value (it's a reply to another msg), but
there is no Reply-ID header field, when converting QWK, look-up the
original message-ID (to use for the relpy/vote Reply-ID value). If the original
message doesn't have a message-ID, use the normal auto-generation scheme.
This isn't normally necessary, but I posted a poll with a message-ID on
DOVE-Net / Sysops and then voted on the poll, creating a vote with no Reply-ID
which causes an SMB "writing" error (access=-105) on the QWKnet node BBSes
(because the required header field is missing).