Telnet not available in v3.7

The telnet command was somehow removed in version 3.7. Was that intentional, and if not, can that be fixed?

Works in 3.6:
```
➜ docker run -it alpine:3.6
/ # telnet
BusyBox v1.26.2 (2017-11-23 08:40:54 GMT) multi-call binary.

Usage: telnet [-a] [-l USER] HOST [PORT]

Connect to telnet server

-a Automatic login with $USER variable
-l USER Automatic login as USER
```

Doesn't work in 3.7:
```
➜ docker run -it alpine:3.7
/ # telnet
/bin/sh: telnet: not found
```