Preserve file descriptor blocking mode in add_reader and add_writer

[HyunsikParker]

Fixes #712.

libuv's uv_poll_init() switches the supplied file descriptor to nonblocking mode. uvloop left that change in place after add_reader() or add_writer(), while the asyncio selector loop preserves the original mode.

This records the mode before initializing the poll handle and restores it once initialization finishes. If restoring it fails, the initialized handle is closed before the error is raised.

The regression test covers reader and writer registration and checks the descriptor both while registered and after removal. The same test runs against uvloop and the asyncio reference loop.

Tests:

• .venv/bin/python -m unittest -v tests.test_sockets (35 passed, 1 skipped)
• With test_fs_event_rename excluded, 528 tests passed with PYTHONASYNCIODEBUG=1 and again with PYTHONASYNCIODEBUG=0 (39 skipped in each run)
• The full suite also exposed the existing intermittent macOS test_fs_event_rename failure. Isolated reruns produced one pass and one CHANGE versus RENAME failure.