Add support for `SEIZE`, `INTERRUPT`, `LISTEN` requests

[ranweiler]

Right now, the nix crate only provides ptrace::seize().

We could implement the others via raw calls to libc. It'd be better to wait / help nix grow support for these calls, include the missing PTRACE_EVENT_STOP variant of Event.

[ranweiler]

nix now has interrupt(), too.