Fix getdomainname for SunOS

[gwr]

The tests (System.Net.NetworkInformation.Functional.Tests) being used to validate the work in #124728 need getdomainname() to work or else the test will abort and dump core. This fixes that for SunOS.

[Copilot]

Pull request overview

Fixes getdomainname() availability on SunOS so System.Net.NetworkInformation.Functional.Tests can run without aborting, by correctly detecting and linking the required SunOS libraries and adding a missing declaration.

Changes:

• Update CMake configuration to detect getdomainname on SunOS via check_function_exists while linking against socket/nsl.
• Add an explicit getdomainname prototype for SunOS builds where no header declares it.
• Link System.Native against libnsl on SunOS.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/native/libs/configure.cmake	SunOS-specific getdomainname detection with required link libraries and restoration of CMAKE_REQUIRED_LIBRARIES.
src/native/libs/System.Native/pal_networking.c	Adds SunOS-only getdomainname forward declaration when the function is available.
src/native/libs/System.Native/extra_libs.cmake	Ensures System.Native links socket and nsl on SunOS.

[rzikm]

LGTM, Thanks!

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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[rzikm]

/ba-g test failure is unrelated