Fixes #154

This should only be used when letsencrypt.sh is under control by a
different script which makes sure that no two processes are touching the
same files.

With this change private and public files are now separated by default.

In case curl gives an error, it's helpful to know the URL
being tried and the method. In the GET case, one can easily
retry in the shell and debug this.

This is implemented by defining ${DOMAINS_TXT} in config.sh. If not
set in config.sh, it defaults to the previously (hard-coded) location,
which is ${BASEDIR}/domains.txt.

Update wellknown.md

apache config that would work for 1.3, 2.0, 2.2 and 2.4

This reverts commit 4dc99533.

I have no idea why I thought reusing the keys would be a good idea
when I first started this...

wellknown.md: Clarify WELLKNOWN variable