Bug 10901

Hello, I am trying to escape varaible which will be used in `sed` command but unfortunately it looks like there is some bug in handling parameter expansion in busybox shell.

Expected:
> Bash in Ubuntu:
```sh
$ var="/Users/Documents/name/file"; echo "${var//\//\\/}"
\/Users\/Documents\/name\/file
```

Actual:
> Shell in Alpine:
```sh
$ var="/Users/Documents/name/file"; echo "${var//\//\\/}"
/Users/Documents/name/file
```

It's wierd because parameter expansion with replacement is working for other characters.

> Bash in Ubuntu:
```sh
$ var="/Users/Documents/name/file"; echo "${var//e/E}"
/UsErs/DocumEnts/namE/filE
```
> Shell in Alpine:
```sh
$ var="/Users/Documents/name/file"; echo "${var//e/E}"
/UsErs/DocumEnts/namE/filE
```

Env:
BusyBox v1.27.2 (2017-11-28 16:17:30 GMT) multi-call binary.
Alpine 3.7

Ref:
https://www.gnu.org/software/bash/manual/bashref.html#Shell-Parameter-Expansion