Merge branch 'mc/http-emptyauth-negotiate-fix'

The 'http.emptyAuth=auto' configuration now correctly attempts
Negotiate authentication before falling back to manual credentials.
This allows seamless Kerberos ticket-based authentication without
requiring users to explicitly set 'http.emptyAuth=true'.

* mc/http-emptyauth-negotiate-fix:
  doc: clarify http.emptyAuth values
  t5563: add tests for http.emptyAuth with Negotiate
  http: attempt Negotiate auth in http.emptyAuth=auto mode
  http: extract http_reauth_prepare() from retry paths