Separate flags for copying left column and top row

Separate flags for copying left column and top row

Having a way to specify that the half-brackets extend upwards but not
leftwards avoids all the rendering problems they were previously
suffering from.  And I've spotted the VT100 scan-line characters,
which should be extended to the left but not upwards.  Since the
middle scan-line is unified with a line-drawing character, they really
need to extend leftwards for symmetry, but extending scan-line 1
upwards would be a disaster.

So now there are two flags, JOIN_U and JOIN_L, to specify joining in
each direction.  I've renamed the both-directions flag as JOIN,
because it's not just specific to line-drawing any more.  I might
eventually want a JOIN_D to suppress hinting of the bottom edge of the
character, but I need to do some testing first.