Making 9.2.x backwards compatible with 9.1.x

[bneradt]

In order to make 9.2.x backwards compatible against plugins compiled
with 9.1.x, this PR does the following:

1. Adds back exposed enum values removed in 9.2.x
2. Any added enum values are put at the end of their enumeration lists.
3. Adds back the TSHttpTxnCntl function

[bneradt]

We'll want this for the 9.x trains (currently 9.2.x and master), but not for the 10.x train. That experimental TSHttpTxnCntl should not exist in 10.x. Thus after this is merged, I'll merge up 10-Dev and then revert this patch from there.

[zwoop]

Are you sure we really need that experimental crud? Is someone using it? :-). The Lua feature has been updated already to use the new API and should be in 9.2.x.

Are you sure we really need that experimental crud? Is someone using it? :-). The Lua feature has been updated already to use the new API and should be in 9.2.x.

I vote to remove it in 10.x, but keep it for 9.2.x. Yahoo has an internal plugin that uses the TSHttpTxnCntl function.

Oh, I see. The regression tests try to use the proxy.client.verify.server config via the TS_CONFIG_SSL_CLIENT_VERIFY_SERVER which I put back in to preserve the ABI, but it fails because none of that is implemented now.

@zwoop / @randall : I think we'll have to back out #7639 from 9.2.x. 10.x will be fine, for sure, but I don't think we can remove porxy.client.verify.server and be compatible with the previous 9.x branches.

Oh, I see. The regression tests try to use the proxy.client.verify.server config via the TS_CONFIG_SSL_CLIENT_VERIFY_SERVER which I put back in to preserve the ABI, but it fails because none of that is implemented now.

@zwoop / @randall : I think we'll have to back out #7639 from 9.2.x. 10.x will be fine, for sure, but I don't think we can remove porxy.client.verify.server and be compatible with the previous 9.x branches.

As an alternative to this, I'm updating the InkAPITest.cc to handle the defunct proxy.client.verify.server config. If this seems OK to @randall and @zwoop, then we can move forward with that.

I am still seeing some issue with data types:
https://trafficserver.apache.org/compat_reports/traffic_server/9.1.x_to_master/compat_report.html

I am still seeing some issue with data types:
https://trafficserver.apache.org/compat_reports/traffic_server/9.1.x_to_master/compat_report.html

We talked about this offline, but just to record it here: this will happen on master because the configure.ac defines TS_VERSION_MAJOR as 10, so some enum values and definitions are removed in master. This shouldn't be a problem once this patch is picked back to 9.2.x.

Cherry-picked to v9.2.x