This acts somewhat like g_file_get_relative_path() in that if it is not a
subdirectory of the parent, NULL is returned. Otherwise the relative path
is returned. We can just dive into the substring instead of copying which
is a bonus point.
And expose it via sysprof_document_process_list_mounts() so that when
inspecting processes we can see what binaries were mapped as well as what
the filesystem looked like to locate those mapped paths.
