download_and_compile.sh: make initial cloning of FGData easy and secure

Because cloning FGData from SourceForge using https does not work (this
has been the case for *years*) whereas normal updates of an existing
clone do work, and so does cloning the repository from its official
GitLab mirror using https, this commits implements a special case in
_gitDownload():

  If the assembled clone URL is
  <https://git.code.sf.net/p/flightgear/fgdata>, then ask the user if he
  agrees to automatically clone FGData from GitLab and arrange the
  repository setup to fetch subsequent updates from SourceForge. This
  should be fine because <https://gitlab.com/flightgear/fgdata> is an
  official mirror of FGData. The default answer is thus to do so. It is
  automatically chosen when download_and_compile.sh is called with
  --non-interactive (new option).

This implies that the initial use of download_and_compile.sh is again as
easy as subsequent ones.

This commits also adds the utility function _yes_no_quit_prompt() to
repeatedly ask a question ($1) until a valid answer has been provided.
The function automatically uses the default answer ($2) if
--non-interactive has been passed to download_and_compile.sh. See the
“discussion” at
<https://sourceforge.net/p/flightgear/mailman/message/37017469/>.