ipc: get rid of -ipc_ppid field in favor of fork_gen

ipc: get rid of -ipc_ppid field in favor of fork_gen

We own all the `fork' calls in our codebase, so rely on the
existing $PublicInbox::OnDestroy::fork_gen instead of using `$$'
to call getpid(2) everywhere.  getpid(2) is slower nowadays
since it's always a syscall on modern glibc whereas it was
cached (somewhat unsafely) in the past.