sortrows harmonization with Octave negative-index inputs

[apjanke]

table.sortrows is implemented in terms of sortrows, including passing its options on to the individual variables' sortrows calls. The problem is that table.sortrows declares that it supports all the Matlab sortrows arguments and options. But Octave's sortrows doesn't support that. In particular:

• Octave's sortrows doesn't support MissingPlacement or ComparisonMethod name/val options.
• Octave represents descending column sorts with a negative column index, instead of a separate 'ascend'/'descend' option.

Harmonize this as much as you can. This may include putting in an upstream Octave bug to request support for additional Matlab-compatible calling forms in its native sortrows. Also, decide whether table's sortrows, in particular its column and 'ascend'/'descend' options should primarily match Octave's or Matlab's, or if it's going to try to combine both in its public interface.