Add checks for brokenness in the Steam installation or Steam Runtime

This is totally untested - it compiles but I have never tried it - so you will almost certainly need to squash in some bug fixes. You can change the commit author to yourself and add a Co-authored-by: Simon McVittie, or leave me as the commit author and add a Co-authored-by for yourself, whichever way round you think makes more sense.

Still to do:

• Unit test coverage for SrtSteamIssues. Obviously our autobuilder is not really going to have Steam installed, so I've added srt_system_info_set_test_home_directory() - the idea is that you can create a g_dir_make_tmp(), then populate it with a fake Steam installation (you might want to call out to a shell or python3 script for that rather than writing it in C!), then instantiate a SrtSystemInfo and use it to inspect the fake home directory.

• Fix the inevitable bugs.

• Wire it up to the system-info.c CLI driver.

• Try it on a real machine with a real Steam installation.

• All of that again, but for SrtRuntimeIssues.