checklocks: synchronize proc network sysctls

Read SACK and ephemeral port ranges from the owning stack instead of
per-file caches. Separate proc mounts can cache different values, and a
rejected SACK write can change its cache. The saved stack already
preserves these settings.

Keep the last successfully written forwarding value, since individual
interfaces may have different forwarding settings. Guard it with a mutex
and publish updates only after the stack setter succeeds, leaving failed
writes out of subsequent readback.

Also make the existing TCP buffer-size helper contracts checkable. Their
mutex still serializes requests through the same byte source.

Cover shared readback, rejected SACK and forwarding writes, and
concurrent forwarding readback in the existing proc test target.

Assisted-by: Codex