Add --fah-reconfigure option which makes fahinstall reconfigure the client even if already configured.

This is to address potential user mistakes done at initial configuration.
Require ./fah6 -configonly to exit with signal 15 (expected when successful). Exit in all other cases.
Make download function download to a temp file, then do (atomic) rename.

This is to avoid ending up with partial files shall the download be interrupted.
The downside of this approach is that file name changes (in case of 3xx
redirections) will not be honored (original name will be used).
With "atomic" download function in place (f454639470cb12efe1c099b630f44f67d7858a9a), we can now download fah6 in one go.