Use btclog for determining valid log levels.

This allows levels to be specified by their shorthand versions (CRIT
vs CRITICAL, for example), works with any capitalization, and adds
support for handling the OFF level (previously the least verbose log
level that could be set was CRITICAL).

config.go

@@ -21,6 +21,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/btcsuite/btclog"
 	flags "github.com/btcsuite/go-flags"
 	"github.com/btcsuite/go-socks/socks"
 	"github.com/decred/dcrd/connmgr"
@@ -191,21 +192,8 @@ func cleanAndExpandPath(path string) string {
 
 // validLogLevel returns whether or not logLevel is a valid debug log level.
 func validLogLevel(logLevel string) bool {
-	switch logLevel {
-	case "trace":
-		fallthrough
-	case "debug":
-		fallthrough
-	case "info":
-		fallthrough
-	case "warn":
-		fallthrough
-	case "error":
-		fallthrough
-	case "critical":
-		return true
-	}
-	return false
+	_, ok := btclog.LevelFromString(logLevel)
+	return ok
 }
 
 // supportedSubsystems returns a sorted slice of the supported subsystems for