Make the install script fully sh compliant (e.g. dash or ash)

[dlangBugzillaToGithub]

Seb reported this on 2018-03-24T05:38:28Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=18655

Description

> sh (or the Shell Command Language) is a programming language described by the POSIX standard. It has many implementations (ksh88, dash, ...). bash can also be considered an implementation of sh (see below).

> Because sh is a specification, not an implementation, /bin/sh is a symlink (or a hard link) to an actual implementation on most POSIX systems.

https://stackoverflow.com/questions/5725296/difference-between-sh-and-bash

There's at least ash and dash that we should test for in our CI.