Expressions containing <> do not throw parsing errors

[geographika]

I ran into this issue when checking SLD output from LAYER CLASS EXPRESSIONS.

I'd not thoroughly read the logical expression docs and so had been unaware that NOT expressions in MapServer can be represented by != or ne, but not <>.

The following layer parses fine in MapServer, but the <> gets parsed as MS_TOKEN_COMPARISON_LT and MS_TOKEN_COMPARISON_GT in

MapServer/src/maplayer.c

Line 679 in 24d36f8

case MS_TOKEN_COMPARISON_NE:

    LAYER
      NAME "Test18"    
      TYPE POINT
      CLASS
        EXPRESSION ( [FID] <> 1 AND [FID] <> 10 )
        STYLE
            COLOR 220 0 0
            WIDTH 1
        END
      END
    END

This probably causes strange issues when rendering data, but is more noticeable when generating SLD as it returns:

  <ogc:Filter>
    <ogc:And>
      <ogc:PropertyIsLessThan>
        <ogc:PropertyName>FID</ogc:PropertyName>
        <ogc:Literal>> 1</ogc:Literal>
      </ogc:PropertyIsLessThan>
      <ogc:PropertyIsLessThan>
        <ogc:PropertyName>FID</ogc:PropertyName>
        <ogc:Literal>> 10 </ogc:Literal>
      </ogc:PropertyIsLessThan>
    </ogc:And>
  </ogc:Filter>

Ideally I think the MapServer expression parser should throw an error when reading the Mapfile (or we attempt to support <> in expressions).