Change all returns to exits.  As expected this has no effect since return and exit should give the same results for main routines. But, nevertheless, using exit is a bit clearer as to intent.
Change all returns to exits.  As expected this has no effect
since return and exit should give the same results for main routines.
But, nevertheless, using exit is a bit clearer as to intent.

svn path=/trunk/; revision=12294

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot