use of FOAM_INST_DIR is outdated

From previous versions, had FOAM_INST_DIR to avoid a manual edit of the bashrc,cshrc files. With the newer logic (BASH_SOURCE, lsof) this will now be largely ignored, except as a hard-coded fallback value within bashrc,cshrc.

Propose renaming it in bashrc,cshrc (perhaps back to foamInstall), and removing it from foamExec, foamJob, wmakeScheduler*

Opinions? @andy @Prashant