shortVal needs to return an unsigned value, added MULBLANK support to xlstool

git-svn-id: https://svn.code.sf.net/p/libxls/code/trunk/libxls@163 982eeb1d-6a45-41fa-94c8-6813189e3c3d
shortVal needs to return an unsigned value

git-svn-id: https://svn.code.sf.net/p/libxls/code/trunk/libxls@164 982eeb1d-6a45-41fa-94c8-6813189e3c3d