Introduce new text.dat line:

  MsgHdrBodySeparator
This is display between the msg header text.dat lines and the message body
(previously, just hard-coded to a blank line).
This enables us to now execute showmsgavatar.js much faster than loading
msghdr.asc file, and using it to execute showmsghdr.js.
So... if you're upgrading to this build, you can get rid of msghdr.asc and
get a message viewing performance boost. The results should look the same
(but the message header will display immediately, with no discernable delay).