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.

i3/checknofullscreen.sh

@@ -1,4 +1,5 @@
-#! /bin/bash
+#!/bin/sh
+
 FOCUSED=$(xprop -root _NET_ACTIVE_WINDOW | awk -F' ' '{print $NF}')
 if xprop -id ${FOCUSED} _NET_WM_STATE | grep -q _NET_WM_STATE_FULLSCREEN; then
 	exit 1

i3/lockscreen.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/bash
+#!/usr/bin/env bash
 
 # shellcheck source=.lockvars
 # shellcheck disable=SC1091