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/msg.go

@@ -40,11 +40,17 @@ type MsgRes struct {
 
 	init *engine.Init
 
-	Body           string            `yaml:"body"`
-	Priority       string            `yaml:"priority"`
-	Fields         map[string]string `yaml:"fields"`
-	Journal        bool              `yaml:"journal"` // enable systemd journal output
-	Syslog         bool              `yaml:"syslog"`  // enable syslog output
+	Body     string            `lang:"body" yaml:"body"`
+	Priority string            `lang:"priority" yaml:"priority"`
+	Fields   map[string]string `lang:"fields" yaml:"fields"`
+
+	// Journal should be true to enable systemd journaled (journald) output.
+	Journal bool `lang:"journal" yaml:"journal"`
+
+	// Syslog should be true to enable traditional syslog output. This is
+	// probably going to somewhere in `/var/log/` on your filesystem.
+	Syslog bool `lang:"syslog" yaml:"syslog"`
+
 	logStateOK     bool
 	journalStateOK bool
 	syslogStateOK  bool