Haiku: use errno crate with Haiku support

Disable `linux_ext` in wasm32 and fortanix rustdoc builds.

The `std::os::unix` module is stubbed out when building docs for these
target platforms. The introduction of Linux-specific extension traits
caused `std::os::net` to depend on sub-modules of `std::os::unix`,
which broke rustdoc for the `wasm32-unknown-unknown` target.

Adding an additional `#[cfg]` guard solves that rustdoc failure by
not declaring `linux_ext` on targets with a stubbed `std::os::unix`.

Haiku: add build fix for ssh2-sys

Haiku: use posix_spawn to spawn new processes

Haiku: split-debuginfo is an unstable option

Haiku: add build fix for ssh2-sys

Rust 1.56.0 (no Haiku-specific patches)

Rust 1.55.0 with Haiku-specific patches to build and run on Haiku R1 …

…beta 3

Rust 1.54.0 with Haiku-specific patches to build and run on Haiku R1 …

…beta 3

Rust 1.53.0 with Haiku-specific patches to build and run on Haiku R1 …

…beta 2