redoconf: posix.rc: fix abort when timespec is not available.

Now we define HAS_POSIX=1 if it works, and to blank otherwise, to be
consistent with other rc scripts, but it doesn't abort if we can't find
timespec.

Meanwhile, slightly clarify the error message in rc.sh.

Reported-by: Nathaniel Filardo <nwfilardo@gmail.com>

redoconf/rc.sh

@@ -15,7 +15,7 @@ _rc_exit_check() {
 		echo "Fatal: used redoconf/rc.sh but didn't call rc_include." >&2
 		exit 91
 	elif [ -n "$RC_QUEUE" ]; then
-		echo "Fatal: must call rc_save or rc_undo before ending." >&2
+		echo "Fatal: script exited without rc_save or rc_undo." >&2
 		exit 92
 	fi
 }