Avoid hard-coding the location of bash

On FreeBSD, bash lives in /usr/local/bin/bash. These scripts don't
really depend on bash so could be changed to /bin/sh.

Signed-off-by: Doug Rabson <dfr@rabson.org>

hack/btrfs_installed_tag.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 cc -E - > /dev/null 2> /dev/null << EOF
 #include <btrfs/ioctl.h>
 EOF

hack/btrfs_tag.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 cc -E - > /dev/null 2> /dev/null << EOF
 #include <btrfs/version.h>
 EOF

hack/libdm_tag.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 tmpdir="$PWD/tmp.$RANDOM"
 mkdir -p "$tmpdir"
 trap 'rm -fr "$tmpdir"' EXIT

hack/tree_status.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 STATUS=$(git status --porcelain)