New round of CI improvements/fixes

[still-flow]

Some small fixups:

• exit->quit in gdb cmdlist (funny that it sorta worked regardless)
• remove unnecessary backtrace gdb cmd (the file and line at fault are already reported without it)
• remove mymake output buffering for all platforms (should help with on-the-fly CI logs)
• remove brew update step (to address Add -Wimplicit-fallthrough #69)
• move from to-be-deprecated emscripten container

OUTDATED (see below for details):

Now some stuff in need of more explanation.

Switch to podman instead of docker: this one is up to debate, but several articles state that one is "better" than the other. I'll be honest and say that I'm not really sure whether all that benefits this project whatsoever, but then again we only use docker in the simplest possible way.

Add a helper log-saver workflow: addressing the issue raised by @Quuxplusone in #117, this should be a temporary workaround until Github perhaps introduces a better way of doing this. For now, I had to create a helper workflow that's triggered after the main one completes, and fetches its logs using Github's API, and uploads them as an artifact. It also displays a link back to the main run in the "annotations" section -- it'd be better if the main run could link to the helper workflow run instead, but there's a chicken-and-egg problem: you can only use the API query after the workflow completes, and by that point AFAIK you can't modify its "executive overview" output. Example output. Btw, feel free to scrutinize the hell out of that one -- these rudimentary scripts were hacked together only to the point of working, not to the point of being "correct".

Note that the CI run for this PR won't produce any log archives yet, because the corresponding .yml file has to exist on master.

[Quuxplusone]

LGTM mod my comments. The only really really important one is the first one, about updating .travis.yml with the new Docker image and/or s/docker/podman/ so that it stays in sync with the Github workflow.

[still-flow]

Addressed all Github-related stuff. Now it remains to update Travis and write some reports to GH support (about brew update and log recycle period as well).

[still-flow]

Of course, it has only now occurred to me to check this, but actually build artifacts also expire on a 90-day basis. Oops.

Therefore, either the new helper workflow is entirely useless and has to be nuked, or a new storage solution has to be found. Either way, this'll need some more thought.

[still-flow]

Reports to Github support both done. Will post the responses if and when they occur.

[still-flow]

Updated Travis config (loosely) in sync with the Github one. Also excised the log-saver workflow for now.

[still-flow]

Workflow-cancelling action doesn't seem to function properly. I guess that's another ticket for GH support right there -- asking for an official way to do so.

[still-flow]

So, podman turned out to be slower on GH and downright not working well on Travis. Rolled that back.

In better news, forcing no brew update on Travis seems to have sped up OSX builds somewhat.

[still-flow]

On log expiration: good news is that Github listened (not to imply that we were the only ones to complain). The fly in the ointment is that for free public repos 90 days is the upper retention limit :/. Can't blame them too much I guess: all that storage can add up quickly.

Besides possible endemic workarounds, I wanted to try out Gitlab's reportedly-celebrated CI anyways. Will report if it's any good.

[zenorogue]

The CI has been broken for a long time, which was annoying. I have removed the broken stuff:

• removed TravisCI badges and YML file
• disabled testing the Windows build using gdb
• disabled testing macos-latest (there was some problem during the libraries installation due to deprecation)
• disabled Android tests (seems some problem with the build system?)
• disabled emscripten tests (actually looks like some problem with the source code, but removed anyway for now)

Also fixed some warnings which caused some Linux/Windows checks to fail.