engine: resources, lang: Set resource fields more accurately

There were some bugs about setting resource fields that were structs
with various fields. This makes things more strict and correct. Now we
check for duplicate field names earlier (duplicates due to identical
aliases) and we also don't try and set private fields, or incorrectly
set partial structs.

Most interestingly, this also cleans up all of the resources and ensures
that each one has nicer docs and a clear struct tag for fields that we
want to use in mcl. These are mandatory now, and if you're missing the
tag, then we will ignore the field.

engine/resources/nspawn.go

@@ -58,7 +58,10 @@ type NspawnRes struct {
 
 	init *engine.Init
 
-	State string `yaml:"state"`
+	// State specifies the desired state for this resource. This must be
+	// either `running` or `stopped`.
+	State string `lang:"state" yaml:"state"`
+
 	// We're using the svc resource to start and stop the machine because
 	// that's what machinectl does. We're not using svc.Watch because then we
 	// would have two watches potentially racing each other and producing