syscall: apply the errno allocation fix to other operating systems

The previously-submitted https://go-review.googlesource.com/#/c/6701
didn't include dragonfly, freebsd, nacl, netbsd, openbsd, or solaris.
(or things like darwin/arm or ppc64 or arm64)

So do them all.

Note I had to copy the function into tables_nacl.go. I found that
preferable to creating a new file just to have suitable build
tags. It's likely this function will be mirrored to plan9 and windows
later too, each of the 4 with their own policy of which error values
are common.

The corresponding x/sys CL for this CL is https://golang.org/cl/8190
but it excludes nacl (not in x/sys) and solaris (already broken).

Update Issue #8859

Change-Id: I91902615692b29b69c905edd9e126a26337294f6
Reviewed-on: https://go-review.googlesource.com/8192Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/mksyscall_solaris.pl

@@ -248,7 +248,7 @@ while(<>) {
 
 	if ($do_errno) {
 		$text .= "\tif e1 != 0 {\n";
-		$text .= "\t\terr = e1\n";
+		$text .= "\t\terr = errnoErr(e1)\n";
 		$text .= "\t}\n";
 	}
 	$text .= "\treturn\n";

src/syscall/tables_nacl.go

@@ -335,3 +335,27 @@ var errorstr = [...]string{
 	ENOSHARE:        "No such host or network path",
 	ECASECLASH:      "Filename exists with different case",
 }
+
+// Do the interface allocations only once for common
+// Errno values.
+var (
+	errEAGAIN error = EAGAIN
+	errEINVAL error = EINVAL
+	errENOENT error = ENOENT
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case EAGAIN:
+		return errEAGAIN
+	case EINVAL:
+		return errEINVAL
+	case ENOENT:
+		return errENOENT
+	}
+	return e
+}