libmach: add stubs for Plan 9

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5576080

src/libmach/plan9.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)
+{
+	USED(pid);
+	USED(msg);
+
+	sysfatal("ctlproc unimplemented on Plan 9");
+	return -1;
+}
+
+char*
+proctextfile(int pid)
+{
+	USED(pid);
+
+	sysfatal("proctextfile unimplemented on Plan 9");
+	return nil;
+}
+
+char*
+procstatus(int pid)
+{
+	USED(pid);
+
+	sysfatal("procstatus unimplemented on Plan 9");
+	return nil;
+}
+
+Map*
+attachproc(int pid, Fhdr *fp)
+{
+	USED(pid);
+	USED(fp);
+
+	sysfatal("attachproc unimplemented on Plan 9");
+	return nil;
+}
+
+void
+detachproc(Map *m)
+{
+	USED(m);
+
+	sysfatal("detachproc unimplemented on Plan 9");
+}
+
+int
+procthreadpids(int pid, int *p, int np)
+{
+	USED(pid);
+	USED(p);
+	USED(np);
+
+	sysfatal("procthreadpids unimplemented on Plan 9");
+	return -1;
+}
+
+int 
+nanosleep(const struct timespec *rqtp, struct timespec *rmtp)
+{
+	USED(rqtp);
+	USED(rmtp);
+
+	sysfatal("nanosleep unimplemented on Plan 9");
+	return -1;
+}