Back compatibility.

Back compatibility.

Tab line functions were enclosed in a conditional block for version >=
"700" as they are not available before Vim 7.  Earlier versions still
parse them, however.  Vim 6 was reading the functions in, barfing on the
:for/:endfor blocks and spitting out an error about :return not inside a
function later on.

For Vim 6 the return WASN'T inside a function because the function
wasn't defined.

As a workaround all :for/:endfor blocks have been replaced with
:while/:endwhile blocks.  Vim 6 is now happy.