foamCleanPath may be too aggressive

It only does what it is told to, but while experimenting with different things I hit this interesting one.

With paraview installed by the operating system, it resided under '/usr/bin/paraviewand/usr/lib64/and has/usr/include/paraview`. Setting 'ParaView_DIR=/usr' to find this version didn't work immediately. But after cleaning out my OpenFOAM environment, everything was from the PATH.

The foamCleanPath used on exit has $ParaView_DIR inside of it. Since this was /usr, my `/usr/bin? was removed from the PATH.

It should modified never to clean out standard system paths

• /bin, /sbin, /usr/bin, /usr/sbin, /usr/local/bin, /lib, /lib64, /usr/lib, /usr/lib64