ssh: pass 0 to procctl(2) to operate on self

[emaste]

Summary

As of f833ab9dd187 procctl(2) allows idtype P_PID with id = 0 as a shortcut for the calling process ID. The shortcut also bypasses the p_cansee / p_candebug test (since the process is able to act on itself.)

Use the id = 0 shortcut in ssh so that the self-procctl will succeed.

It's likely the case that procctl(2) should allow the self-operation with getpid(). This change is suitable for the base system where we can be certain we are running on a kernel with f833ab9dd187 (and id = 0 is preferable anyway).

Reported by: Shawn Webb
Sponsored by: The FreeBSD Foundation

[emaste]

Could abandon this change if we change kern_procctl to skip the `error = cmd_info->need_candebug ? p_candebug(td, p) : p_cansee(td, p); in the case that we're operating on ourself.

[kib]

The change in this review is good because it removes one syscall (not too big achievement on its own, but stil a right thing to do). Removing the unneeded calls in kernel are also good IMO, we do not envision that a process can be stripped from debugging permissions to itself [I believe Windows allows such thing].