sftp: add --sftp-skip-links to skip symlinks and non regular files - fixes #3716

This also corrects the symlink detection logic to only check symlink
files.  Previous to this it was checking all directories too which was
making it do more stat calls than was necessary.

backend/sftp/sftp.go

@@ -156,6 +156,11 @@ Home directory can be found in a shared folder called "home"
 			Default:  "",
 			Help:     "The command used to read sha1 hashes. Leave blank for autodetect.",
 			Advanced: true,
+		}, {
+			Name:     "skip_links",
+			Default:  false,
+			Help:     "Set to skip any symlinks and any other non regular files.",
+			Advanced: true,
 		}},
 	}
 	fs.Register(fsi)
@@ -177,6 +182,7 @@ type Options struct {
 	SetModTime        bool   `config:"set_modtime"`
 	Md5sumCommand     string `config:"md5sum_command"`
 	Sha1sumCommand    string `config:"sha1sum_command"`
+	SkipLinks         bool   `config:"skip_links"`
 }
 
 // Fs stores the interface to the remote SFTP files
@@ -600,12 +606,16 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 		remote := path.Join(dir, info.Name())
 		// If file is a symlink (not a regular file is the best cross platform test we can do), do a stat to
 		// pick up the size and type of the destination, instead of the size and type of the symlink.
-		if !info.Mode().IsRegular() {
+		if !info.Mode().IsRegular() && !info.IsDir() {
+			if f.opt.SkipLinks {
+				// skip non regular file if SkipLinks is set
+				continue
+			}
 			oldInfo := info
 			info, err = f.stat(remote)
 			if err != nil {
 				if !os.IsNotExist(err) {
-					fs.Errorf(remote, "stat of non-regular file/dir failed: %v", err)
+					fs.Errorf(remote, "stat of non-regular file failed: %v", err)
 				}
 				info = oldInfo
 			}