feature/taildrop: start moving Taildrop out of LocalBackend

This adds a feature/taildrop package, a ts_omit_taildrop build tag,
and starts moving code to feature/taildrop. In some cases, code
remains where it was but is now behind a build tag. Future changes
will move code to an extension and out of LocalBackend, etc.

Updates #12614

Change-Id: Idf96c61144d1a5f707039ceb2ff59c99f5c1642f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

taildrop/taildrop.go

@@ -18,6 +18,7 @@ import (
 	"path"
 	"path/filepath"
 	"regexp"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -239,6 +240,11 @@ func (m *Manager) IncomingFiles() []ipn.PartialFile {
 		})
 		f.mu.Unlock()
 	}
+
+	sort.Slice(files, func(i, j int) bool {
+		return files[i].Started.Before(files[j].Started)
+	})
+
 	return files
 }