event/kqueue: add EVFILT_MACHPORT

[utzcoz]

Mach ports are the only IPC on Apple platforms that can carry a port right, so anything built on them needs a way to wait on one. kqueue has EVFILT_MACHPORT for that, but there was no way to reach it from here.

Report readiness only, with no fflags: the filter can also dequeue a message into a buffer supplied through the kevent64 extension fields, which this API does not model. Waiting for a message and then receiving it is the common case and needs none of that.

The identifier is a mach_port_name_t rather than a file descriptor, so it gets a type of its own instead of reusing RawFd.

[utzcoz]

Looks like CI failures are not related this PR:

error: attributes starting with `rustc` are reserved for use by the `rustc` compiler
  --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.4/src/backend/linux_raw/io/errno.rs:49:25
   |
49 | #[cfg_attr(rustc_attrs, rustc_layout_scalar_valid_range_start(0xf001))]
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: attributes starting with `rustc` are reserved for use by the `rustc` compiler
  --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.4/src/backend/linux_raw/io/errno.rs:50:25
   |
50 | #[cfg_attr(rustc_attrs, rustc_layout_scalar_valid_range_end(0xffff))]
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: cannot find attribute `rustc_layout_scalar_valid_range_start` in this scope
  --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.4/src/backend/linux_raw/io/errno.rs:49:25
   |
49 | #[cfg_attr(rustc_attrs, rustc_layout_scalar_valid_range_start(0xf001))]
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: cannot find attribute `rustc_layout_scalar_valid_range_end` in this scope
  --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-1.1.4/src/backend/linux_raw/io/errno.rs:50:25
   |
50 | #[cfg_attr(rustc_attrs, rustc_layout_scalar_valid_range_end(0xffff))]
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^