Fix multicolumn regression (bug #9788)

Fix multicolumn regression (bug #9788)

The test case did show several problems:
- The alignment argument was not parsed correctly if it was not in braces
- There one column too much created, since I did not take into account that
  the current cell must bge replaced by the multicolumn cell
- If the last line of an array contained only an empty multicolumn cell, then
  the complete multicolumn was swallowed
- The decision whether to output the column separator & was sometimes wrong
  for multicolumns