os: os.ExpandEnv() inconsistent behavior with $- due to missing - as shell special parameter

[cuonglm]

- is one of shell special parameters.

Function isShellSpecialVar() missed it so $- and ${-} is expanded inconsistently with os.ExpandEnv().

---

1. What version of Go are you using (go version)?
   go version go1.6.3 linux/amd64

2. What operating system and processor architecture are you using (go env)?

   GOARCH="amd64"
   GOBIN=""
   GOEXE=""
   GOHOSTARCH="amd64"
   GOHOSTOS="linux"
   GOOS="linux"
   GOPATH="/home/cuonglm/go"
   GORACE=""
   GOROOT="/home/cuonglm/sources/go"
   GOTOOLDIR="/home/cuonglm/sources/go/pkg/tool/linux_amd64"
   GO15VENDOREXPERIMENT="1"
   CC="gcc"
   GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
   CXX="g++"
   CGO_ENABLED="1"
   

3. What did you do?
   https://play.golang.org/p/EfqG4gN1so

4. What did you expect to see?

   <nil>
   dash true
   dash
   dash
   

5. What did you see instead?

   <nil>
   dash true
   -
   dash
   

[gopherbot]

CL https://golang.org/cl/25352 mentions this issue.

[cuonglm]

@quentinmit Would you mind reviewing https://golang.org/cl/25352 ?