CHANGE OF THE DECADE: I finally gave m_unknown() a name: thisisanmbox().

CHANGE OF THE DECADE: I finally gave m_unknown() a name: thisisanmbox().
The ``magic invocation'' of this function informs m_getfld() that it is
reading a packed mbox file and not an MH-style (one-file-per-message) file.
m_getfld() needs to care for message delimiters then. The ``magic'' of
m_unknown() had vanished some time ago, already. Back then, it needed to
guess which kind of mailbox format the file is. With dropping the support
for MMDF (^A^A^A^A) maildrops, we converted m_unknown() into a plain dump
function ... and now, it also got a boring name. Well ...