Use debuginfod server(s) in Linux crash catcher

Sometimes, crash logs from users contain less information than they could because they're running stripped binaries for OpenMW and all its dependencies. Obviously, it's unreasonable to ask all users to proactively install debug packages just in case they hit a crash. We don't need to do that, though, as (provided one exists) a debuginfod can supply the debug symbols to the debugger as and when they're needed.

For typical desktop Linux distros these days, a debuginfod server is available, and partially set up out of the box by setting the DEBUGINFOD_URLS environment variable to list that distro's server, but isn't actually active because debuginfod is disabled by default in GDB.

https://debuginfod.elfutils.org (documented here) is also available as a server that federates the servers for several distros.

I think we should adjust the Linux crash catcher so it shows a popup (or checks an environment variable or something so it works headless), and if the user agrees, sets DEBUGINFOD_URLS to A_LIST_THAT_CAN_BE_SET_VIA_CMAKE $DEBUGINFOD_URLS https://debuginfod.elfutils.org and includes set debuginfod enabled on in the script given to GDB to produce the crash log.

We need the CMake-controllable list because there might be a server with symbols for binaries that aren't from distro packages, e.g. we have symbols for the new generic Linux builds and their dependencies available via https://openmw-sym.rgw.ctrl-c.liu.se, just like we do for our Windows builds. We don't want to just hardcode that URL in because it does no one any good to check that URL for symbols it definitely doesn't provide.