Make signal handling portable (#122)

Plan9port and Plan 9 acme handles these signal:

```
char *oknotes[] ={
	"delete",
	"hangup",	// SIGHUP
	"kill",		// SIGTERM
	"exit",
	nil
};
```

I'm not sure what generates the "delete" and "exit" signals, but SIGTERM
was missing from Edwood's list, so I've added that. SIGQUIT is useful
for debugging a Go program (typing ctrl-\ will print stack trace of all
running goroutines), so I removed that. SIGSTOP cannot be handled, so
that was also removed.

Edwood was ignoring all other signals, which seems wrong. I made it so
it'll only ignore what plan9port acme was ignoring:

```
char *ignotes[] = {
	"sys: write on closed pipe",	// SIGPIPE
	"sys: ttin",	// SIGTTIN
	"sys: ttou",	// SIGTTOU
	"sys: tstp",	// SIGTSTP
	nil
};
```

Helps #115
Helps #116

acme.go

@@ -11,7 +11,6 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
-	"syscall"
 	"time"
 
 	"9fans.net/go/draw"
@@ -175,7 +174,8 @@ func main() {
 	go newwindowthread()
 	go xfidallocthread(display)
 
-	signal.Notify(csignal /*, hangupsignals...*/)
+	signal.Ignore(ignoreSignals...)
+	signal.Notify(csignal, hangupSignals...)
 	for {
 		select {
 		case <-cexit:
@@ -665,29 +665,17 @@ func killprocs() {
 
 var dumping bool
 
-var hangupsignals = []os.Signal{
-	os.Signal(syscall.SIGINT),
-	os.Signal(syscall.SIGHUP),
-	os.Signal(syscall.SIGQUIT),
-	os.Signal(syscall.SIGSTOP),
-}
-
 // TODO(rjk): I'm not sure that this is the right thing to do? It fails to
 // handle the situation that is most interesting: trying to save the state
 // if we would otherwise crash. It's also conceivably racy.
 func shutdown(s os.Signal) {
-	for _, sig := range hangupsignals {
-		if sig == s {
-			if !dumping && os.Getpid() == mainpid {
-				killprocs()
-				dumping = true
-				row.Dump("")
-			} else {
-				os.Exit(0)
-			}
-		}
+	if !dumping && os.Getpid() == mainpid {
+		killprocs()
+		dumping = true
+		row.Dump("")
+	} else {
+		os.Exit(0)
 	}
-	return
 }
 
 type errorWriter struct{}

fsys.go

@@ -114,7 +114,7 @@ func fsysproc() {
 	for {
 		fc, err := plan9.ReadFcall(sfd)
 		if err != nil || fc == nil {
-			acmeerror("fsysproc: ", err)
+			acmeerror("fsysproc", err)
 		}
 		if x == nil {
 			cxfidalloc <- nil

signal_plan9.go

@@ -0,0 +1,18 @@
+// +build plan9
+
+package main
+
+import (
+	"os"
+	"syscall"
+)
+
+var ignoreSignals = []os.Signal{
+	syscall.Note("sys: write on closed pipe"),
+}
+
+var hangupSignals = []os.Signal{
+	syscall.SIGINT,
+	syscall.SIGTERM,
+	syscall.SIGHUP,
+}

signal_unix.go

@@ -0,0 +1,21 @@
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package main
+
+import (
+	"os"
+	"syscall"
+)
+
+var ignoreSignals = []os.Signal{
+	syscall.SIGPIPE,
+	syscall.SIGTTIN,
+	syscall.SIGTTOU,
+	syscall.SIGTSTP,
+}
+
+var hangupSignals = []os.Signal{
+	syscall.SIGINT,
+	syscall.SIGTERM,
+	syscall.SIGHUP,
+}

signal_windows.go

@@ -0,0 +1,18 @@
+// +build windows
+
+package main
+
+import (
+	"os"
+	"syscall"
+)
+
+var ignoreSignals = []os.Signal{
+	syscall.SIGPIPE,
+}
+
+var hangupSignals = []os.Signal{
+	syscall.SIGINT,
+	syscall.SIGTERM,
+	syscall.SIGHUP,
+}