If unspecified then a sensible default is used.

Valid values: 
```
aes128-ctr
aes192-ctr
aes256-ctr
******@****.***
******@****.***
******@****.***
arcfour256
arcfour128
arcfour
aes128-cbc
3des-cbc
```

`--sftp=mac-algos=...` specifies a default set of MAC algorithms in preference order.