This is needed to support overriding these options inside match
blocks. A new option `programs.ssh.extraOptionOverrides` has been
added to allow global overrides.
This module generates a `.ssh/config` file. This doesn't embed _all_
options for the ssh client, but the most common ones should be there.
Example usage:
```nix
programs.ssh = {
enable = true;
forwardAgent = true;
controlMaster = "auto";
matchBlocks = [
{
host = "something.blah.edu";
port = 1024;
user = "cleague";
identitiesOnly = true;
}
{
host = "host1 host2 host2.net host2.com";
port = 7422;
hostname = "example.com";
serverAliveInterval = 60;
}
{
host = "lucian";
forwardX11 = true;
forwardX11Trusted = true;
checkHostIP = false;
};
};
};
```
Each entry in `programs.ssh.matchBlocks` must contain a `host` field,
which will be used for the block condition.
