patch from div0:

patch from div0:
fix console weirdnesses:
* empty lines kill command history (instead don't add empty lines to the history)
* cursor position and history line length were done the wrong way, positioning
  the cursor outside of the line or only recalling part of a line

NOTE: maybe it is better to move the Con_Printf BELOW the return so an empty
prompt isn't even displayed? bash does not do it like that, IRC clients do it
like that, and personally I don't care

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6552 d7cf8633-e32d-0410-b094-e92efae38249