Make multicast-group helpers and vsock fallback portable to QNX

[dallison]

Summary

Two small portability fixes that let cpp_toolbelt compile on QNX 8 (Neutrino) while staying buildable on Linux, macOS and BSDs. Pairs with the QNX port of subspace: dallison/subspace#62.

This PR is draft because it has only been reviewed statically — it has not yet been built on a real QNX target.

toolbelt/sockets.cc — UDPSocket::JoinMulticastGroup / LeaveMulticastGroup

Previously used the Linux-only struct ip_mreqn (with imr_ifindex). On macOS, BSDs and QNX, only the POSIX-portable struct ip_mreq is available, so this code failed to compile there.

Switched to struct ip_mreq. Behaviour is unchanged on Linux: the previous code passed imr_ifindex = 0, which selects the default interface; ip_mreq.imr_interface = INADDR_ANY does the same.

toolbelt/sockets.h — sockaddr_vm fallback

The fallback definition of struct sockaddr_vm (used when neither <linux/vm_sockets.h> nor <sys/vsock.h> is available, e.g. on QNX) had a typo defined(_APPLE__) (single leading underscore). This is harmless on Apple, where the system header is used instead, but on a system that actually hits the fallback a true __APPLE__ macro would never have inserted svm_len. Fixed the spelling.

Notes

• Subspace doesn't currently call JoinMulticastGroup / LeaveMulticastGroup itself, so this is purely a "unblock the build" fix; no runtime behaviour change is observable from subspace.
• VSOCK at runtime still cannot work on QNX (no AF_VSOCK socket family); the fallback sockaddr_vm definition only exists to keep code that references the type compilable.

Test plan

• bazel test //... on Linux to confirm ip_mreq behaves identically to the previous ip_mreqn use (multicast join / leave still work).
• bazel test //... on macOS to confirm the multicast helpers now compile (previously they would have failed to build there too).
• On a host with a QNX toolchain: qcc build of sockets.cc succeeds.
• If we have a multicast test, confirm a full round-trip of Join → sendto → recvfrom → Leave works on Linux.

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