Make do_home() more useful in softwrap mode: let it move to the beginning
of the current chunk instead of to the beginning of the whole line; only
when already at the beginning of a chunk, let it move to the beginning of
the line.  This is called "dynamic home'.

The above rules are ignored when --smarthome is in effect and the cursor
is somewhere in the leading whitespace of a line -- then the cursor is
moved to the first non-whitespace character of the line.