flag, which specifies external or resource intensive tests to
perform.  This is used by test_largefile and test_socket_ssl.

-u/--use takes a comma separated list of flags, currently supported:
largefile, network.

usage(): New function.  Note that the semantics of main() have changed
    slightly; instead of returning an error code, it raises a
    SystemExit (via sys.exit()) with the given error code.

main(): use_large_resources => use_resources
    Also, added support for long-option alternative to the short
    options.

_expectations: Added test_socket_ssl to the list of expectedly skipped
    tests.