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/user.go

@@ -47,13 +47,29 @@ type UserRes struct {
 
 	init *engine.Init
 
-	State             string   `yaml:"state"`             // state: exists, absent
-	UID               *uint32  `yaml:"uid"`               // uid must be unique unless AllowDuplicateUID is true
-	GID               *uint32  `yaml:"gid"`               // gid of the user's primary group
-	Group             *string  `yaml:"group"`             // name of the user's primary group
-	Groups            []string `yaml:"groups"`            // list of supplemental groups
-	HomeDir           *string  `yaml:"homedir"`           // path to the user's home directory
-	AllowDuplicateUID bool     `yaml:"allowduplicateuid"` // allow duplicate uid
+	// State is either exists or absent.
+	State string `lang:"state" yaml:"state"`
+
+	// UID specifies the usually unique user ID. It must be unique unless
+	// AllowDuplicateUID is true.
+	UID *uint32 `lang:"uid" yaml:"uid"`
+
+	// GID of the user's primary group.
+	GID *uint32 `lang:"gid" yaml:"gid"`
+
+	// Group is the name of the user's primary group.
+	Group *string `lang:"group" yaml:"group"`
+
+	// Groups are a list of supplemental groups.
+	Groups []string `lang:"groups" yaml:"groups"`
+
+	// HomeDir is the path to the user's home directory.
+	HomeDir *string `lang:"homedir" yaml:"homedir"`
+
+	// AllowDuplicateUID is needed for a UID to be non-unique. This is rare
+	// but happens if you want more than one username to access the
+	// resources of the same UID. See the --non-unique flag in `useradd`.
+	AllowDuplicateUID bool `lang:"allowduplicateuid" yaml:"allowduplicateuid"`
 
 	recWatcher *recwatch.RecWatcher
 }