runtime: handle non-nil-terminated environment strings on Plan 9

Russ Cox pointed out that environment strings are not
required to be nil-terminated on Plan 9.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/159130044

src/runtime/env_plan9.go

@@ -30,7 +30,7 @@ func gogetenv(key string) string {
 	if fd < 0 {
 		return ""
 	}
-	n := seek(fd, 0, 2) - 1
+	n := seek(fd, 0, 2)
 	if n <= 0 {
 		close(fd)
 		return ""
@@ -44,6 +44,10 @@ func gogetenv(key string) string {
 		return ""
 	}
 
+	if p[r-1] == 0 {
+		r--
+	}
+
 	var s string
 	sp := (*_string)(unsafe.Pointer(&s))
 	sp.str = &p[0]