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/group.go

@@ -45,8 +45,11 @@ type GroupRes struct {
 
 	init *engine.Init
 
-	State string  `yaml:"state"` // state: exists, absent
-	GID   *uint32 `yaml:"gid"`   // the group's gid
+	// State is `exists` or `absent`.
+	State string `lang:"state" yaml:"state"`
+
+	// GID is the group's gid.
+	GID *uint32 `lang:"gid" yaml:"gid"`
 
 	recWatcher *recwatch.RecWatcher
 }