refactor(protocols): one home per duplicated RFC-fixed constant (#1726 ruling 7) #1739

Is your feature request related to a problem? Please describe.
Ruling 7 on #1726: RFC-fixed protocol constants that are duplicated across modules get one home, the module that owns the protocol, and they are never made configurable. The config-module PR leaves this for a follow-up, because both edit the same toolkit files.

Duplicates found by #1726's worker and its review (lines on d06701064):

• toolkit/pypcapfile.py:85,94,97: TCP_MIN_HEADER_LEN = 20, IPV4_HEADER_LEN, IPV6_HEADER_LEN = 40
• toolkit/pcap.py:178: a bare 20
• foundation/reassembly/ipv6.py:27: _IPV6_HDR_LEN = 40
• foundation/reassembly/ip.py:35: _MAX_LENGTH = 65535
• the µs/ns resolution literals, the PCAP and PCAP-NG magics, and the PCAP-NG minimum-block tables

Describe the solution you'd like
Inventory every duplicate, give each a single home in the owning protocol module, and point readers at that home. Removing an old name is breaking and gets a versionremoved directive.

Blocked until
The #1726 config PR merges.