Restore the sys_o58_kill system call.

Restore the sys_o58_kill system call.

By keeping both the new (sys_kill/sys_thrkill) and old (sys_o58_kill)
system calls for the OpenBSD 6.0 release, code that uses either of these
mechanisms will work on both of the supported OpenBSD releases. This
provides a clean transition for runtimes that make direct system calls
(namely the Go programming language).

This requires a minimal amount of non-intrusive code and does not block
development progress within OpenBSD.

ok deraadt@ guenther@