standardize the default shell

I do not know of a need to have the default shell be a build-time
configurable setting. All *nix systems I am aware of have /bin/sh as a
default posix compatible shell.
If some systems running OpenRC do not make that assumption about
/bin/sh, I will consider bringing this back, so feel free to open an
issue.

scripts/halt.in

@@ -1,4 +1,4 @@
-#!@SHELL@
+#!/bin/sh
 
 option_arg=
 poweroff_arg=

scripts/poweroff.in

@@ -1,4 +1,4 @@
-#!@SHELL@
+#!/bin/sh
 
 option_arg=
 poweroff_arg=

scripts/rc-sstat.in

@@ -1,4 +1,4 @@
-#!@SHELL@
+#!/bin/sh
 # Copyright (c) 2015 The OpenRC Authors.
 # See the Authors file at the top-level directory of this distribution and
 # https://github.com/OpenRC/openrc/blob/master/AUTHORS

scripts/reboot.in

@@ -1,4 +1,4 @@
-#!@SHELL@
+#!/bin/sh
 
 option_arg=
 poweroff_arg=

scripts/shutdown.in

@@ -1,4 +1,4 @@
-#!@SHELL@
+#!/bin/sh
 
 shutdown_arg=
 while getopts :akrhPHfFnct: opt; do