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/mount.go

@@ -109,13 +109,24 @@ type MountRes struct {
 
 	init *engine.Init
 
-	// State must be exists ot absent. If absent, remaining fields are ignored.
-	State   string            `yaml:"state"`
-	Device  string            `yaml:"device"`  // location of the device or image
-	Type    string            `yaml:"type"`    // the type of filesystem
-	Options map[string]string `yaml:"options"` // mount options
-	Freq    int               `yaml:"freq"`    // dump frequency
-	PassNo  int               `yaml:"passno"`  // verification order
+	// State must be exists or absent. If absent, remaining fields are
+	// ignored.
+	State string `lang:"state" yaml:"state"`
+
+	// Device is the location of the device or image.
+	Device string `lang:"device" yaml:"device"`
+
+	// Type of the filesystem.
+	Type string `lang:"type" yaml:"type"`
+
+	// Options are mount options.
+	Options map[string]string `lang:"options" yaml:"options"`
+
+	// Freq is the dump frequency.
+	Freq int `lang:"freq" yaml:"freq"`
+
+	// PassNo is the verification order.
+	PassNo int `lang:"passno" yaml:"passno"`
 
 	mount *fstab.Mount // struct representing the mount
 }