(invoke_main_func): Don't call exit here. Throws that

(invoke_main_func): Don't call exit here.  Throws that
are only caught by scm_with_guile will bypass us and would cause
scm_boot_guile to return erroneously.
(scm_boot_guile): Expect scm_with_guile to return and call exit
here, passing it an appropriate exit code.