http: fix handling of missing CURLPROTO_*

http: fix handling of missing CURLPROTO_*

Commit aeae4db1 refactored the handling of the curl protocol
restriction support into a function but failed to add a version
check for older versions of curl that lack CURLPROTO_* support.

Add the missing check and at the same time convert it to a feature
check instead of a version based check.  This is done to ensure that
vendor supported curl versions that have had CURLPROTO_* support
backported are handled correctly.

Signed-off-by: Tom G. Christensen <tgc@jupiterrise.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>