Added `sigaction_is_default()`, `sigaction_is_ignore()`, and `sigaction_current()` functions

[cpick]

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What does this PR do

Provide safe mechanisms to determine whether a signal's action is the default or ignore. This covers the majority of real-world calls to sigaction() with a NULL action without needing any unsafe code.

Provide an unsafe way to query the currently installed sigaction.

The decision to add sigaction_current() instead of just exposing the (new) sigaction_inner() function was to avoid any confusion over the semantics of passing in a None sigaction argument (eg: someone thinking that it meant remove or reset the action).

This closes #2172

Checklist:

• [x] I have read CONTRIBUTING.md
• [x] I have written necessary tests and rustdoc comments
• [x] A change log has been added if this PR modifies nix's API

[cpick]

I've also added a (loosely) related/complementary proposal to add safe functions to ignore and restore signals' actions in #2191.