various fixes:

- enhanced port installation
- added jail.conf variable ports=YYYYMMDD
- fixed build and install bugs
- exit on mount error
- check existence of src+dst before mounting
- only remove stuff in a new base in non-ports mode (!rw)
- fixed fetch runner

README.md

@@ -146,10 +146,16 @@ Then install the build jail as usual:
 
 `jaildk build myjail`
 
-Finally, install the current ports collection:
+Install the current ports collection:
 
 `jaildk fetch`
 
+In case the  ports version created does not match  the version of your
+jail, you need  to configure the different ports version  in your jail
+config `/jail/etc/myjail/jail.conf` like this:
+
+`ports=20201127`
+
 Now you can enter the build jail and install ports the traditional way:
 
 ```