BUG/MEDIUM: cache: ignore cache on redundant origin/referer

BUG/MEDIUM: cache: ignore cache on redundant origin/referer

The cache's Vary header processing to consider secondary keys used only
the first occurrence of Origin and Referer. These header are single-valued
so this is not supposed to be a problem, however the server's behavior
when dealing with more than one will not necessarily be predictable, so
we'd rather just bypass the cache when facing such requests.

default_normalizer() now returns -1 if a second occurrence is found,
making http_request_build_secondary_key() return a failure and never
match from secondary keys. This way it doesn't affect the standard case
and doesn't take risks with border-line ones.

The vary regtest was completed with various combinations.

Reported-by: Claude (ANT-2026-FQEGP4YG)
This should be backported to all stable versions.