Make the init script a portable POSIX shell script

This translates init-tests-after-clone.sh from bash to POSIX sh,
changing the hashbang and replacing all bash-specific features, so
that it can be run on any POSIX-compatible ("Bourne style") shell,
including but not limited to bash. This allows it to be used even
on systems that don't have any version of bash installed anywhere.

Only that script is modified. The other shell scripts make greater
use of (and gain greater benefit from) bash features, and are also
only used for building and releasing. In contrast, this script is
meant to be used by most users who clone the repository.

init-tests-after-clone.sh

@@ -1,12 +1,16 @@
-#!/usr/bin/env bash
+#!/bin/sh
 
 set -e
 
-if [[ -z "$TRAVIS" ]]; then
-    read -rp "This operation will destroy locally modified files. Continue ? [N/y]: " answer
-    if [[ ! $answer =~ [yY] ]]; then
-        exit 2
-    fi
+if test -z "$TRAVIS"; then
+    printf 'This operation will destroy locally modified files. Continue ? [N/y]: ' >&2
+    read -r answer
+    case "$answer" in
+    [yY])
+        ;;
+    *)
+        exit 2 ;;
+    esac
 fi
 
 git tag __testing_point__