Start converting user mark to only be displayed when set.
Current code will crash if commands that need the user mark are called
and it is not set.
Move umark conversion done.
Step one... we should never crash because the mark is not set.
However, once we set the mark.. (except in Znext_error) we never clear
it.
Started adding the obvious clears.
Just in case the mark and the point are on the same line best to set
Tlrow in vsetmrk itself.