lib/rand: Break out random functions into separate package

The intention for this package is to provide a combination of the
security of crypto/rand and the convenience of math/rand. It should be
the first choice of random data unless ultimate performance is required
and the usage is provably irrelevant from a security standpoint.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3186

lib/config/config.go

@@ -19,6 +19,7 @@ import (
 	"strings"
 
 	"github.com/syncthing/syncthing/lib/protocol"
+	"github.com/syncthing/syncthing/lib/rand"
 	"github.com/syncthing/syncthing/lib/util"
 )
 
@@ -254,7 +255,7 @@ func (cfg *Configuration) prepare(myID protocol.DeviceID) {
 	}
 
 	if cfg.GUI.APIKey == "" {
-		cfg.GUI.APIKey = util.RandomString(32)
+		cfg.GUI.APIKey = rand.String(32)
 	}
 }