[dev.cc] runtime: convert Plan 9 port to Go

Thanks to Aram Hăvărneanu, Nick Owens
and Russ Cox for the early reviews.

LGTM=aram, rsc
R=rsc, lucio.dere, aram, ality
CC=golang-codereviews, mischief
https://golang.org/cl/175370043
diff --git a/src/runtime/signal_plan9.go b/src/runtime/signal_plan9.go
new file mode 100644
index 0000000..37d2435
--- /dev/null
+++ b/src/runtime/signal_plan9.go
@@ -0,0 +1,54 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+type sigTabT struct {
+	flags int
+	name  []byte
+}
+
+// Incoming notes are compared against this table using strncmp, so the
+// order matters: longer patterns must appear before their prefixes.
+// There are _SIG constants in os2_plan9.go for the table index of some
+// of these.
+//
+// If you add entries to this table, you must respect the prefix ordering
+// and also update the constant values is os2_plan9.go.
+var sigtable = [...]sigTabT{
+	// Traps that we cannot be recovered.
+	{_SigThrow, []byte("sys: trap: debug exception")},
+	{_SigThrow, []byte("sys: trap: invalid opcode")},
+
+	// We can recover from some memory errors in runtime·sigpanic.
+	{_SigPanic, []byte("sys: trap: fault read addr")},  // SIGRFAULT
+	{_SigPanic, []byte("sys: trap: fault write addr")}, // SIGWFAULT
+
+	// We can also recover from math errors.
+	{_SigPanic, []byte("sys: trap: divide error")}, // SIGINTDIV
+	{_SigPanic, []byte("sys: fp:")},                // SIGFLOAT
+
+	// All other traps are normally handled as if they were marked SigThrow.
+	// We mark them SigPanic here so that debug.SetPanicOnFault will work.
+	{_SigPanic, []byte("sys: trap:")}, // SIGTRAP
+
+	// Writes to a closed pipe can be handled if desired, otherwise they're ignored.
+	{_SigNotify, []byte("sys: write on closed pipe")},
+
+	// Other system notes are more serious and cannot be recovered.
+	{_SigThrow, []byte("sys:")},
+
+	// Issued to all other procs when calling runtime·exit.
+	{_SigGoExit, []byte("go: exit ")},
+
+	// Kill is sent by external programs to cause an exit.
+	{_SigKill, []byte("kill")},
+
+	// Interrupts can be handled if desired, otherwise they cause an exit.
+	{_SigNotify + _SigKill, []byte("interrupt")},
+	{_SigNotify + _SigKill, []byte("hangup")},
+
+	// Alarms can be handled if desired, otherwise they're ignored.
+	{_SigNotify, []byte("alarm")},
+}