os400: Disable Alt-Svc by default since it's experimental

Follow-up to 520f0b47 which added Alt-Svc support and enabled it by
default for OS400. Since the feature is experimental, it should be
disabled by default.

Ref: https://github.com/curl/curl/commit/520f0b4#commitcomment-32792332
Ref: https://curl.haxx.se/mail/lib-2019-02/0008.html

Closes https://github.com/curl/curl/pull/3688