libmach: stubs for openbsd

Add libmach stubs for openbsd.

R=rsc
CC=golang-dev
https://golang.org/cl/4815065

src/libmach/openbsd.c

+// This is stubbed out for the moment. Will revisit when the time comes.
+#include <u.h>
+#include <libc.h>
+#include <bio.h>
+#include <mach.h>
+
+int
+ctlproc(int pid, char *msg)
+{
+	sysfatal("ctlproc unimplemented in OpenBSD");
+	return -1;
+}
+
+char*
+proctextfile(int pid)
+{
+	sysfatal("proctextfile unimplemented in OpenBSD");
+	return nil;
+}
+
+char*
+procstatus(int pid)
+{
+	sysfatal("procstatus unimplemented in OpenBSD");
+	return nil;
+}
+
+Map*
+attachproc(int pid, Fhdr *fp)
+{
+	sysfatal("attachproc unimplemented in OpenBSD");
+	return nil;
+}
+
+void
+detachproc(Map *m)
+{
+	sysfatal("detachproc unimplemented in OpenBSD");
+}
+
+int
+procthreadpids(int pid, int *p, int np)
+{
+	sysfatal("procthreadpids unimplemented in OpenBSD");
+	return -1;
+}