The code for the linux distro scanner and its tests got lgtm'd, but we're not actually using it in mongoc-metadata.c. Once we get the code for OS version merged in, refactor mongoc-metadata.c to call _mongoc_linux_distro_scanner_get_distro, to set the OS name correctly under linux.

I'm filing this as a separate ticket because just getting the linux distro scanner itself in has been pretty complicated, and I don't want to combine that with semi-big changes to the other metadata gathering code.