fix(mh): 24 BitField flag namespaces leave reserved bits unnamed, so they rebuild as zero #1418

Describe the bug
24 MH BitField namespaces leave bits unnamed, and the unnamed bits are packed as zero on rebuild. #1416's invariant test found them (start, length):

• Messages: BindingRevocationMessage (3,13), FastBindingAcknowledgmentMessage (1,7), FastBindingUpdateMessage (4,12), FlowBindingMessage (1,7), HandoverAcknowledgeMessage (3,5), HandoverInitiateMessage (4,4), HeartbeatMessage (0,14), LocalizedRoutingAcknowledgmentMessage (1,7), SubscriptionResponseMessage (1,7), UpdateNotificationMessage (2,14)
• Options: DNSUpdateOption (1,7), DelegatedMNPOption (1,7), DynamicIPMulticastSelectorOption (1,7), IPv4AddressAcknowledgementOption (6,2), IPv4DHCPSupportModeOption (0,15), IPv4HomeAddressReplyOption (6,2), IPv4HomeAddressRequestOption (6,10), IPv4TrafficOffloadSelectorOption (1,31), MAGMultipathBindingOption (2,22), NATDetectionOption (1,15), RedirectOption (2,14), TransientBindingOption (0,7)
• Other: MultiPrefixExtension (1,31), PerSessionBitRateAttribute (2,14)

All are .flags fields.

Expected behavior
Byte-exact, per the every-round-trip ruling. Name reserved at each site, carry it through the reader, data model and make(), then delete the entries from KNOWN_GAPS in #1416's test. Related to #1329 (MH reserved fields, blocked).

Part of #1288.