libfetch: parse scheme://domain:/ correctly

This improves URL-parsing compability with cURL, and unbreaks parsing of
similar kinds of URLs after commit 8d9de5b10a24.

Sponsored by: Juniper Networks, Inc.
Reviewed by: des
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44493

(cherry picked from commit fb860ed0c52c2c1e7792ef86718620a439663c7f)