ENH: improve bash completion functionality (issue #551)

- use complete -o filenames, dropped -o nospace to make it more responsive.

- restructure completion code to use a unified backend, which makes it easier
  understand, maintain and re-use.

- foamCreateBashCompletions now simply outputs to a stdout, and allows
  quick generation of completion of single applications.

- add -fileHandler completion in anticipation of future changes there.

- relocated as etc/config.s/bash_completion to prevent inadvertently
  having two versions (.com, .org) installed at the same time.