Submessage 5 - Post a message

Client Message.

BYTE[4] BoardID
BYTE[4] Replying to ID (0 if this is a top level / non-reply post)
BYTE subjectLen  (length of the subject, includes null termination)
BYTE[subjectLen] subject (null terminated)
BYTE numlines
For each line:
        BYTE linelen
        BYTE[linelen] body (null terminated)