rpc : make RDMA probe diagnostics more verbose
Log the target GID (local address) being matched against, the
GGML_RDMA_DEV / GGML_RDMA_GID env overrides if set, and every GID
present on each probed device (with its type and a [MATCH] marker).
This makes it clear why a device is skipped when RDMA is not usable.
Assisted-by: llama.cpp:DeepSeek-4-Flash-0731
