update comments for command_version

Based on information in #2969.
The `listdir` command requires slave version 2.16
only use obfuscated passwords on command_version 2.16 and higher

Fixes #2969.
check the slave version of a more sensible command
Merge pull request #1292 from djmitche/bug2969

Correctly handle obfuscation with older slaves

(a separate PR needs to be submitted for eight)


Fixes ticket:2969