unpicking dns packets in strace'd programs

[John Hughes]

> This patch adds support -edns=DESCRIPTORS to strace; the listed
> descriptors will have any data read or written on them interpreted
> as a DNS packet, and the content of the packets parsed and included
> in the output.

> +#define QUAL_DNS 0400 /* parse data read/written as UDP DNS
>                           packets */

Wouldn't it be nice if:

1. we had a pointer to a decoder func for each fd, so we
   could add extra formats easily.

2. we could automaticaly decide what the format was, e.g.
   if it's port 53 it's probably DNS?