Make sysctl_doeproc() more predictable

Swap the order of looking into zombie and all process lists, start now
with the zombie one. This prevents a race observed previously that the
same process could be detected on both lists during a single polling call.

While there:
- Short-circuit break for KERN_PROC_PID, once a pid has been detected.
- Removal of redundant "if (kbuf)" and "if (marker)" checks.
- Update of comments regarding potential optimization, explaining why we
  don't want to it as of now. Performance gain from lookup call vs
  iteration over a list is neglible on a regular system.
- Return ESRCH when no results have been found. This allows more easily
  to implement a retry or abandon algorithm.

This corrects races observed in the existing ATF ptrace(2) tests, related
to await_zombie(). This function was expecting to check whether a process
has been transformed into a zombie, however it was causing occasional
crashes as it was overflowing the return buffer, returning the same pid
twice: once from allproc list and the second time from zombieproc one.

Fix suggested by <christos>
Short-circuit break suggested by <kre>

Discussed on tech-kern.

Sponsored by <The NetBSD Foundation>

(kamil)