isatty_plan9.go: initial commit (#42)

Based on the best-guess behavior of ape's isatty for Plan 9.

isatty_plan9.go

diff --git a/isatty_plan9.go b/isatty_plan9.go
new file mode 100644
index 0000000..1c14999
--- /dev/null
+++ b/isatty_plan9.go
@@ -0,0 +1,15 @@
+// +build plan9
+
+package isatty
+
+import (
+	"syscall"
+)
+
+func IsTerminal(fd uintptr) bool {
+	path, err := syscall.Fd2path(fd)
+	if err != nil {
+		return false
+	}
+	return path == "/dev/cons" || path == "/mnt/term/dev/cons"
+}