Support for disabling invocation of rescue mode from kernel cmdline

The rescue mode that can be invoked from the kernel command line is
potentially unsafe.  Many systems lock the root user account, or use
another account for managing, e.g. 'admin'.  The sulogin program(s)
would on such systems give the user a root prompt.

In #357 we added support for setting a custom sulogin user, but for some
systems that is not enough.  On many embedded systems the /etc/passwd
and shadow files are populated at bootstrap and at the time rescue mode
runs, these files will be unpopulated.

The only, truly safe, approach on such systems is to disable rescue mode
completely.  Otherwise intricate Finit plugins have to be used that run
before rescue mode is started -- increasing the complexity of the system
as a whole.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

configure.ac

@@ -71,7 +71,7 @@ AC_ARG_ENABLE(redirect,
 	enable_redirect=yes])
 
 AC_ARG_ENABLE(logrotate,
-        AS_HELP_STRING([--disable-logrotate], [Disable built-in rotation of /var/log/wtmp, default enabled]),,[
+        AS_HELP_STRING([--disable-logrotate], [Disable built-in rotation of /var/log/wtmp]),,[
 	enable_logrotate=yes])
 
 AC_ARG_ENABLE(doc,
@@ -82,6 +82,10 @@ AC_ARG_ENABLE(contrib,
         AS_HELP_STRING([--disable-contrib], [Disable build and install of contrib section]),,[
 	enable_contrib=yes])
 
+AC_ARG_ENABLE([rescue],
+	AS_HELP_STRING([--disable-rescue], [Disable potentially unsafe rescue mode]),,
+	[enable_rescue=yes])
+
 # Check for extra plugins to enable
 AC_ARG_ENABLE(all_plugins,
     AS_HELP_STRING([--enable-all-plugins], [Enable all plugins, default: auto]),
@@ -185,6 +189,9 @@ AS_IF([test "x$enable_redirect" = "xyes"], [
 AS_IF([test "x$enable_logrotate" != "xno"], [
 	AC_DEFINE(LOGROTATE_ENABLED, 1, [Enable built-in rotation of /var/log/wtmp et al.])])
 
+AS_IF([test "x$enable_rescue" != "xno"], [
+	AC_DEFINE([RESCUE_MODE], 1, [Define to enable support for rescue mode.])])
+
 AM_CONDITIONAL(LOGROTATE, [test "x$enable_logrotate" = "xyes"])
 
 ### With features ##############################################################################
@@ -355,6 +362,7 @@ Optional features:
   Skip fsck check.......: $enable_fastboot
   Run fsck fix mode.....: $enable_fsckfix
   Redirect output.......: $enable_redirect
+  Rescue mode...........: $enable_rescue
   Default hostname......: $hostname
   Default group.........: $group
   Default runlevel......: $runlevel