mysqld_safe: use sh not bash

c902d5a4 changed this to bash.
This isn't approprate as mysqld_safe is used by FreeBSD where
there is no bash by default.

The original reason to use bash seems related to MDEV-3279 which
was a dash bug fixed in 2009 - https://git.kernel.org/pub/scm/utils/dash/dash.git/commit/?id=3800d4934391b144fd261a7957aea72ced7d47ea

Working around 9 year old fixed bugs shouldn't be done here.

scripts/mysqld_safe.sh

-#!/bin/bash
+#!/bin/sh
 # Copyright Abandoned 1996 TCX DataKonsult AB & Monty Program KB & Detron HB
 # This file is public domain and comes with NO WARRANTY of any kind
 #