The RFC 42 settings option derives the Host/Match line from the
attribute name. That makes it impossible to express headers that carry
Nix string context (e.g. Match exec referring to a store path), and
forces long computed patterns into attribute names where a stable
logical name would be nicer for dag ordering.
Add an explicit `header` option on each block that defaults to the
attribute name (with the existing Host/Match prefix detection), and
have the legacy matchBlocks shim populate it instead of the
__hmSshBlockHeader internal.
16663cb138