Fix LinkageMap.findConsumers() to check whether the master link for a lib

Fix LinkageMap.findConsumers() to check whether the master link for a lib
providing a given soname actually points to that lib. If there is another
version of this lib with the same soname and the master link points to
that other version, this lib will be shadowed and won't have any consumers.
By eliminating false, positives this way, we avoid the following state
after upgrade from media-libs/mesa-7.0.3 to media-libs/mesa-7.1_rc3:

# scanelf -S /usr/lib64/libGLU.so*
 TYPE   SONAME FILE
ET_DYN libGLU.so.1 /usr/lib64/libGLU.so
ET_DYN libGLU.so.1 /usr/lib64/libGLU.so.1
ET_DYN libGLU.so.1 /usr/lib64/libGLU.so.1.3
ET_DYN libGLU.so.1 /usr/lib64/libGLU.so.1.3.070003
ET_DYN libGLU.so.1 /usr/lib64/libGLU.so.1.3.070100 <- shadowed lib

Thanks to Diego "Flameeyes" Pettenò for reporting this issue.

svn path=/main/trunk/; revision=11200