Save the original content of ~/.ssh/config before parsing the file,
to reduce the risk of introducing hidden bugs while parsing.

Previously the GHKG section was always moved at the end of the file. But
the order counts in ssh_config, so this was breaking configs were a
"Host *" section in present at the end of the file (to define default
settings): the section was moved before GHKG, so it would break our
config.

The parsing of ~/.ssh/config is completely reworked to:
- perserve the position of an existing GHKG section.
- inject a new GHKG section at the end of the file, except if the last
  section is "Host *" (in that case, put it just above)
- recognize a "Host" section in lower case

The code is not enabled for now as I'm not sure it is really useful,
while I'm sure it could be annoying (if you have set a passphrase to the
newly created key, it will be asked).

... unless $ENV{GITHUB_KEYGEN_DOWNGRADE}