Use EXIT_SUCCESS and EXIT_FAILURE

Use EXIT_SUCCESS and EXIT_FAILURE

This is a bit of C pedantry.  POSIX requires that an exit status of
zero is success and non-zero is failure, but the C standard says no
such thing.  Instead it provides EXIT_SUCCESS and EXIT_FAILURE.  Since
Bedstead aspires to work on non-POSIX systems, it should use the
defined constants.  Also removing magic numbers is a good thing.