ENH: improvements to foamConfigurePaths tool (#1010)

- provide edit for adjusting the ThirdParty and fallback site variables.
  This is useful when packaging into a single directory, and possibly
  without any ThirdParty dependencies.

- support '-prefix' (similar to auotconfig) as an alternative to
  '-foamInstall'. Use this prefix to set the prefix directly.
  Bypasses and disables the bash sourcing magic.