net: a refused thread spawn drops one peer, not the node

std::thread throws std::system_error when pthread_create is refused
(RLIMIT_NPROC, a container's pids cgroup, memory pressure), and nothing
caught it, so the node died by std::terminate. Threads are spawned per
peer connection, so the pressure that triggers this scales with peer
count, which is attacker-influenced. Observed killing a healthy
participant worker mid-mining.

Every spawn site in the node build, and what a failure now does:

- btftunnel.cpp: both tunnel pumps are guarded. The inbound pump starts
  first because it owns cleanup of the pump end and rendezvous socket;
  if the second spawn is refused, shutdown() wakes the first out of its
  read and it cleans up itself -- closing those descriptors from the
  failure path instead would race it onto reused descriptors. Both
  paths return INVALID_SOCKET, which every caller already treats as
  "this connection failed".
- btfrv.cpp: the relay's accept loop drops the connection it cannot
  serve and stops accepting for a moment -- longer after consecutive
  failures, capped at 5s -- instead of dying at an arbitrary one. The
  per-pair second pump now runs on the handling thread itself, leaving
  exactly one spawn to guard and nothing running yet to race.
- main.cpp: a participant that cannot spawn its hash workers stops the
  ones that did start, drops the pool connection, and returns to the
  reconnect loop, the same way an ordinary disconnect does.
- randomx_pow.cpp: dataset init joins the workers it got, releases the
  dataset, and stays in light mode -- the same trade the allocation
  failure path above it already makes.
- rpc.cpp, net.cpp: the remaining unchecked _beginthread calls (which
  report failure by value, they cannot throw) now say precisely what
  did not start. The pool's accept and payout threads failing silently
  left the pool running deaf -- nobody accepts, nobody gets paid, and
  nothing in the log says why.

Left alone deliberately: the per-relay accept spawns (rpc.cpp) already
check, log and retry on the next poll; startup-critical spawns in
net.cpp keep failing hard; the selftest-only std::thread uses may abort
a selftest run, which is acceptable where dying over one peer is not.

Verified: full node rebuilds clean and links; pool-stratum selftest
passes; a scratch harness under RLIMIT_NPROC=1 confirms _beginthread
reports -1 and std::thread throws system_error, the premises of every
path above.

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