shell: add more interpreters (#3708)

* ash: only interpreter, extension is more commonly used for
  Kingdom of Loathing scripting, e.g. github.com/twistedmage/assorted-kol-scripts

* dash: only interpreter, extension is more commonly used for
  dashboarding-related stuff

* ksh: extension was already present

* mksh

* pdksh

test/fixtures/Shell/ash-env

@@ -0,0 +1,2 @@
+#!/usr/bin/env ash
+echo "ash"

test/fixtures/Shell/dash-env

@@ -0,0 +1,2 @@
+#!/usr/bin/env dash
+echo "dash"

test/fixtures/Shell/ksh-env

@@ -0,0 +1,2 @@
+#!/usr/bin/env ksh
+echo "ksh"

test/fixtures/Shell/mksh

@@ -0,0 +1,2 @@
+#!/bin/mksh
+echo "mksh"

test/fixtures/Shell/pdksh-env

@@ -0,0 +1,2 @@
+#!/usr/bin/env pdksh
+echo "pdksh"