unset csh environment variables when sourcing cshrc twice

Summary

In a c-shell if sourcing ~OpenFOAM/OpenFOAM-v2112/etc/cshrc MORE THAN ONCE, the OpenFOAM paths are missing from the environment variable $PATH and so OpenFOAM code will no longer operate.

Steps to reproduce

> source ~OpenFOAM/OpenFOAM-v2112/etc/cshrc
> echo $PATH     
> source ~OpenFOAM/OpenFOAM-v2112/etc/cshrc 
> echo $PATH

Example case

What is the current bug behaviour?

OpenFOAM paths are missing from $PATH

What is the expected correct behavior?

OpenFOAM paths are in $PATH

Relevant logs and/or images

Environment information

OpenFOAM version :v2112 (and possibly earlier)
Operating system : centos or any (presumably)
Hardware info :
Compiler :gcc or any (presumably)

Possible fixes

In etc/config.csh/setup there are two occurrences of the following three lines:

_foamClean PATH "$foamOldDirs"
_foamClean MANPATH "$foamOldDirs"
_foamClean -lib "$foamOldDirs"

If you replace the second occurrence by

_foamClean PATH
_foamClean MANPATH
_foamClean -lib

This fixes the issue.