v.mkgrid should offer breaks option for vertical lines too

[marisn]

Issue mentioned in user ML by Hamish.

Design decision needed - should breaks be used for both horizontal and vertical breaks or it should be renamed to h_breaks and v_breaks?

Original message: 20deg reprojected grid lines are a bit ugly, apparently we need to extend the v.mkgrid breaks= option to work on vertical lines as well as the horizontal ones.  http://lists.osgeo.org/pipermail/grass-user/2012-July/065214.html

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