Implement parsing for supported -o flags

[kontsevoy]

For better compatibility with OpenSSH we shall consider certain subset of -o options to be passed to tsh ssh. We already have support for many and at the very least we should consider accepting and ignoring options which are on by default. The candidates include:

-o AddKeysToAgent
-o ForwardAgent
-o RequestTTY
-o StrictHostKeyChecking

Related issue: #2217

Details

-o AddKeysToAgent should be fully supported. The default behavior is "yes" but if a user specifies "no" we should NOT add keys to the agent.

-o ForwardAgent should be full supported similarly to above.

-o RequestTTY is basically an alias to -i so it should be fully supported.

-o StrictHostKeyChecking should be fully supported. This one is interesting because it makes no sense in a standard Teleport deployment (because all hosts are authenticated) but it does make sense when Teleport is used with OpenSSH (and yes, we ask a user if they trust a node).

If any other OpenSSH option is used (see man ssh) it should result in a WARNING written to stderr saying WARNING: option XXX is not supported but otherwise tsh should proceed.

If anything else (not a valid OpenSSH option) is used with -o, it should result in ERROR written to stderr saying ERROR: option XXX is not supported and tsh must exit with an error exit status/code (probably 1?)

[aaraujo-nydig]

hi @kontsevoy @russjones, did this ever got into a release? I'm trying the latest tsh version v3.1.4 and I'm getting error: unknown short flag '-o'.

[kontsevoy]

@russjones re-opening because I can reproduce the problem too.

[kontsevoy]

@russjones also, when we figure out what's going on, let's assign it to me to update the docs? Thanks.

[kontsevoy]

I am getting:

# This is what aaraujo-nygig sees, because -o is passed to 'tsh' instead of 'tsh ssh'
$ tsh -o ForwardAgent=yes ssh t1-master
error: unknown short flag '-o'


# does not parse ForwardAgent
$ tsh ssh -o ForwardAgent=yes t1-master
error: invalid format for option

# same issue
$ tsh ssh -o RequstTTY=yes t1-master
error: invalid format for option

For the 1st issue, it would be nice to generate a friendlier user message (maybe say "perhaps you want to use this flag as tsh ssh -o?", not sure if our command line parser allows customizing error messages for misplaced flags). For the 2nd issue it's unclear why wouldn't it work.