[$] Exceptions in BPF

The BPF virtual machine in the kernel has been steadily gaining newfeatures for years, many of which add capabilities that C programmers donot ordinarily have. So, from one point of view, it was only a matter oftime before BPF gained support for exceptions. As it turns out, though,this "exceptions" feature is aimed at a specific use case, and its use inmost programs will be truly exceptional.