build-sys: don't mke use of "sushell" automatically

"sushell" is a Fedora-specific concept, shipped as part of
"initscripts". We shouldn't actively search for it if we can avoid it.
Hence, lets now default to /bin/sh as debug shell on all systems, and
permit Fedora to override that for their RPMs via --with-debug-shell= at
configure time.

NEWS

@@ -2,6 +2,11 @@ systemd System and Service Manager
 
 CHANGES WITH 233 in spe
 
+        * The shell invoked by debug-shell.service now defaults to /bin/sh in
+          all cases. If distributions want to use a different shell for this
+          purpose (for example Fedora's /sbin/sushell) they need to specify
+          this explicitly at configure time using --with-debug-shell=.
+
         * The confirmation spawn prompt has been reworked to offer the
           following choices:
 

configure.ac

@@ -531,8 +531,8 @@ AS_IF([test "x$enable_wheel_group" != "xno"], [
 AC_ARG_WITH(debug-shell,
         AS_HELP_STRING([--with-debug-shell=PATH],
                 [path to debug shell binary]),
-        [SUSHELL="$withval"],[
-        AS_IF([test "x${have_selinux}" != "xno"], [SUSHELL="/sbin/sushell"] , [SUSHELL="/bin/sh"])])
+        [SUSHELL="$withval"],
+        [SUSHELL="/bin/sh"])
 
 AC_SUBST(SUSHELL)