Handle pselect signal masks

[jaybosamiya-ms]

This PR fixes the type of SigSetPack to actually use the pointer (see the pselect6 part of man 2 pselect), and adds support for its use as a temporary signal mask.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure trait_requires_more_generic_type_params: trait now requires more generic type parameters ---

Description:
A trait now requires more generic type parameters than it used to. Uses of this trait that supplied the previously-required number of generic types will be broken. To fix this, consider supplying default values for newly-added generic types.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-parameter-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/trait_requires_more_generic_type_params.ron

Failed in:
  trait SigSetPack (0 -> 1 required generic types) in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1857

--- failure type_requires_more_generic_type_params: type now requires more generic type parameters ---

Description:
A type now requires more generic type parameters than it used to. Uses of this type that supplied the previously-required number of generic types will be broken. To fix this, consider supplying default values for newly-added generic types.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-parameter-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/type_requires_more_generic_type_params.ron

Failed in:
  Struct SigSetPack (0 -> 1 required generic types) in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1857

[CvvT]

LGTM, thanks!