* caddyhttp: restore allow_underscore_in_headers server option (#7808)
* caddyhttp: mark insecure_allow_underscore_in_headers as EXPERIMENTAL
* caddyhttp: replace underscore bool with expected_underscore_headers allowlist
* fix gofmt alignment in serveroptions.go
* caddyhttp: drop repeated allowlisted underscore headers
* caddyhttp: add tests for repeated-value drop and variant-drop logging