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.
