Fix cross-platform compilation (Windows, macOS) and resolve buffer overflows for modern I2P destinations

[jolavillette]

This Pull Request consolidates multiple critical fixes to make libsam3 fully cross-platform, secure, and compatible with the modern I2P network.

🛠️ Key Changes

1. Cross-Platform Compilation (Windows & macOS)

• Windows compatibility: Fixed header inclusion order to prevent conflicts (e.g., <winsock2.h> vs <windows.h>). Corrected socket types (SOCKET vs int) and format strings for proper MinGW/MSVC support.
• Non-blocking sockets: Replaced Unix-specific fcntl(..., F_SETFL, O_NONBLOCK) with the correct ioctlsocket(..., FIONBIO, ...) implementation on Windows.
• macOS support: Resolved compilation and linkage errors specific to Apple Clang and Darwin environments.

2. Security & Modern I2P Destinations

• Addressed critical buffer overflows that occurred when handling modern, larger I2P destination keys (which have grown due to newer crypto like Ed25519).
• Increased static buffer sizes and strengthened boundary checks to prevent memory corruption and ensure safe communication with current I2P routers.

🧪 Testing

• Windows: Compilation verified using MSYS2/MinGW. Network sockets operate correctly in non-blocking mode.
• macOS: Compilation verified using Apple Clang.
• Linux: Standard build remains fully functional without regressions.