sftpd: add support for some SSH commands

md5sum, sha1sum are used by rclone.
cd, pwd improve the support for RemoteFiles mobile app.

These commands are all implemented inside SFTPGo so they work even
if the matching system commands are not available, for example on Windows

sftpgo.json

@@ -18,7 +18,8 @@
     "ciphers": [],
     "macs": [],
     "login_banner_file": "",
-    "setstat_mode": 0
+    "setstat_mode": 0,
+    "enabled_ssh_commands": ["md5sum", "sha1sum", "cd", "pwd"]
   },
   "data_provider": {
     "driver": "sqlite",