Use DtdProcessing for a fresh mutable XML reader settings local

Replace standalone ProhibitDtd assignments on a closed fresh XmlReaderSettings local with the equivalent Prohibit/Parse DtdProcessing value. Retain boolean evaluation and XML policy. Require direct configuration assignments before reader creation and references to be BCL settings properties or a known two-argument XmlReader.Create use; skip borrowed/read-only settings, assignment values, escapes/captures, reassignment, async/iterators, directives and snapshots. Read-only setter diagnostics differ, so no general receiver substitution is made.