Adapt shebangs

Convert all shebangs to /usr/bin/env bash, if bash is really required in
the script. Otherwise convert it to /bin/sh.

/bin/bash is not available on all systems, while /usr/bin/env bash and
/bin/sh are.

bash/bashrc

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 shopt -s autocd
 shopt -s checkwinsize