netstat: model legacy TcpExt counters removed from recent kernels

[neoLsH]

What

Add 10 legacy TcpExt counters to the /proc/net/netstat model and parser:
PAWSPassive, TCPPrequeued, TCPDirectCopyFromBacklog,
TCPDirectCopyFromPrequeue, TCPPrequeueDropped, TCPLoss,
TCPFACKReorder, TCPForwardRetrans, TCPHPHitsToUser,
TCPSchedulerFailed.

Why

These counters are absent from the current kernel's TcpExt table (none of
them are in linus/master include/uapi/linux/snmp.h), but older kernels still
print them in /proc/net/netstat. node_exporter is switching its netstat
collector to these parsers (prometheus/node_exporter#3796), and would
otherwise silently drop these metrics for users whose
--collector.netstat.fields regex selects them.

Behavior

• Purely additive: the fields are *float64 and stay nil when the kernel
  does not print them (all modern kernels), matching the optional-field
  semantics from Replace float64 with *float64 for proc_netstat, proc_snmp and proc_snmp6 #464.
• Tests cover both the "older kernel prints them" and "modern kernel doesn't"
  cases.

Out of scope

• IcmpMsg / Icmp6Msg per-type counters: the kernel only prints non-zero
  InTypeN/OutTypeN entries, so the set is host-dependent and open-ended —
  a fixed struct can't cover it (a map, like NetDev, or an unknown-keys
  escape hatch would). Separate follow-up.
• The StatValue refactor floated in netstat: Use procfs parsers for netstat, snmp and snmp6 node_exporter#3796: changing
  all netstat/snmp field types would be a breaking API change for existing
  consumers; separate discussion.

Note on scope: this PR closes the backward gap (counters older kernels print that
were never modeled). procfs's TcpExt also lags the current kernel in the other
direction (e.g. TCPDelivered, TCPAckCompressed, TCPZeroWindowDrop,
TCPBacklogCoalesce, BeyondWindow, TCPTimeoutRehash, TCPDSACKRecvSegs,
TCPMigrateReq*, TCPAO* are printed by modern kernels but not modeled). That
forward lag is procfs's ordinary upkeep, pre-exists this change, and affects all
consumers; it is deliberately out of scope here rather than bundled in.

[SuperQ]

This needs a DCO sign-off. You can use git commit -s --amend to add it.