(usage): Use EXIT_SUCCESS, not 0, for clarity.

(usage): Use EXIT_SUCCESS, not 0, for clarity.
(main): Initialize exit_failure to EXIT_FAIL.
(main): Use initialize_exit_failure rather than
setting exit_failure directly; this optimizes away redundant
assignments.
(main): Use EXIT_ENOENT and EXIT_CANNOT_INVOKE
rather than roll-your-own symbols or integers.
(main): Exit with status 1, not 2, on errors detected by env proper.