ENH: use new foamEtcFile options to simplify syntax when sourcing files

Can now use this:
    _foamSourceEtc config.sh/scotch
    _foamSourceEtc config.csh/scotch

instead of this:
    _foamSource $($WM_PROJECT_DIR/bin/foamEtcFile config.sh/scotch)
    _foamSource `$WM_PROJECT_DIR/bin/foamEtcFile config.csh/scotch`

In the bash/sh version, leave the _foamSource function for now, since
ThirdParty is still relying on it.

STYLE: elminate while-loop for _foamAddPath etc since this type of
construct isn't readily possible for csh and isn't being used anywhere.