Fix Mach port leak in alt_clock_gettime on macOS

[qingweizhang054]

Description:

This PR fixes a Mach port resource leak in the macOS implementation of alt_clock_gettime.

On macOS versions prior to 10.12 (Sierra, released in 2016), clock_gettime is not available, libzmq falls back to alt_clock_gettime, which uses Mach APIs to retrieve timing information. The current implementation calls mach_host_self() to obtain a host port, but does not release the associated send right after use.

Since mach_host_self() returns a send right that must be explicitly deallocated, repeated calls to alt_clock_gettime in long-running processes can lead to Mach port leaks and eventual resource exhaustion.

This change ensures that the host port returned by mach_host_self() is properly released via mach_port_deallocate() after use. The fix is limited to the legacy macOS code path and does not affect platforms that use the native clock_gettime implementation.

Impact:

• Prevents Mach port leaks on older macOS systems
• Improves stability for long-running applications
• No behavior change on modern macOS or other platforms

[qingweizhang054]

Note: CI reports failures on the GitHub Actions macOS runner in
tests/test_zmq_poll_fd and tests/test_zmq_ppoll_fd, where sendto()
returns EHOSTUNREACH (errno 65).

I cannot reproduce this locally on macOS, and this PR does not modify any
networking-related code paths. This appears to be an environment-specific
issue on the macOS runner rather than a regression introduced by this
change.

Relevant CI output:

FAIL: tests/test_zmq_poll_fd
sendto(...) failed, errno = 65 (Host unreachable)

FAIL: tests/test_zmq_ppoll_fd
sendto(...) failed, errno = 65 (Host unreachable)

[qingweizhang054]

This PR is ready for review when convenient.
It fixes a resource leak in the macOS timing fallback and is limited
to the legacy macOS code path.

Thanks for your time.
@bluca