A first step towards refactoring the module.
Unfortunately the config is not consistent in using comma-separated
values or repeated keys for lists, since the user can always add commas
by themselves, we should default to repeated keys in the generator.
Make use of the recently added nullable `mkPackageOption` feature
to disable installing an SSH client by default: most people should use
the client provided by their system.
* ssh: add generic Match support for matchBlocks
Introduce conservative support for actual `Match`
blocks in ssh config.
"Conservative" means this PR doesn'tt try to process
the `match` expression and simply uses it as a string
provided by the user.
If set, `match` has precedence over `host` meaning
if both are set, `match` is used and `host` is ignored.
* Add news entry
This switches the type of `matchBlocks` from `loaOf` to `listOrDagOf`.
The former has been deprecated in Nixpkgs. The latter allows
dependencies between entries to be expressed using the DAG functions.
