#36038 util: remove usage of `strerror_s/r`

• Simplify operating system error code formatting across platforms
• Remove custom platform branching and build configuration checks

Replaces platform-specific strerror_r, strerror_s, and Win32 FormatMessageA error string logic in SysErrorString with standard C++ std::system_category().message(err). It also drops Win32ErrorString by routing callers to SysErrorString, and removes the STRERROR_R_CHAR_P CMake introspection check and the corresponding linter exception.

Problem: The node previously maintained custom multi-platform branching (POSIX vs. GNU strerror_r, Windows strerror_s, and Win32 FormatMessageA) to safely format OS error codes, requiring dedicated CMake configuration probes.

P4 · cleanup

• P4 because it is an internal cleanup with no performance, stability, or user-visible impact
• Simplifies cross-platform error string handling while keeping behavior effectively unchanged

P4 because it is an internal code simplification with no stated performance, stability, or user-facing impact. It collapses platform-specific formatting branches into std::system_category().message, but leaves SysErrorString behavior effectively unchanged.

Membership: Directly refactors error formatting utilities in src/util/syserror.{cpp,h} and consolidates Win32 error formatting.

Factors: security/stability 0, bug 0, performance 0, user value 0, leverage 0

• Ready for review: passes CI and has no unresolved questions

Branch is rebased, passes CI, and has no unresolved questions.

Author status: active

Resolved concerns:

• Comment indentation style in syserror.h was addressed per a nit from maflcko.

• No substantive reviews or concept comments yet
• Resolved a documentation formatting nit (maflcko)

No substantive reviews or concept comments yet; one styling nit resolved (maflcko).

The only comment is a documentation formatting nit from maflcko that was addressed, and nits do not count as overall reviews.

• maflcko left a formatting nit on syserror.h doxygen comments, which fanquake resolved in a subsequent push.

Review verdicts (DrahtBot): 0

1 lines under test/bench/ci.

• src/util/syserror.cpp +2/-40
• cmake/introspection.cmake +0/-12
• src/util/syserror.h +2/-8
• cmake/bitcoin-build-config.h.in +0/-3
• src/util/fs.cpp +1/-1
• src/util/fs_helpers.cpp +1/-1
• src/util/sock.cpp +1/-1
• test/lint/lint-locale-dependence.py +0/-1

PR #36038 simplifies system error formatting by replacing custom platform-specific wrappers around strerror_r, strerror_s, and Win32 FormatMessageA with std::system_category().message(err). This consolidates Win32ErrorString into SysErrorString and removes an obsolete CMake introspection probe. It is a minor internal utility cleanup with no behavioral or performance impact. The PR is ready for review with clean CI, but has not received substantive reviews yet.