fix: drop privileges to original user when running with sudo

[blink-so]

Problem

When jail is executed with sudo, the subprocess runs as root instead of the original user:

sudo jail -- whoami  # Returns "root" instead of original username

This causes issues because:

• Tools and applications behave differently when run as root
• User-specific configurations and paths are not accessible
• Security implications of running user code as root

Solution

This PR implements privilege dropping to make the subprocess run as the original user when jail is executed with sudo.

Implementation

Clean and Minimal Approach:

• No environment variable manipulation
• No complex path restoration
• Just fixes the core issue: subprocess user identity

Linux (network/linux.go):

• Detects sudo execution via SUDO_UID/SUDO_GID environment variables
• Uses syscall.Credential to set subprocess UID/GID
• Proper error handling with fallback to root if parsing fails

macOS (network/macos.go):

• Same privilege dropping logic as Linux
• Preserves original group behavior for non-sudo execution
• Maintains compatibility with existing macOS jail functionality

Usage

# Before: subprocess runs as root
sudo jail --allow "*" -- whoami  # Returns "root"

# After: subprocess runs as original user
sudo jail --allow "*" -- whoami  # Returns your username

Testing

The implementation has been tested to ensure:

• ✅ Subprocess runs as original user when using sudo
• ✅ Normal execution (without sudo) unchanged
• ✅ Proper error handling for invalid SUDO_UID/SUDO_GID
• ✅ Cross-platform compatibility (Linux and macOS)

Benefits

• Simple: Minimal code change, focused on the core issue
• Safe: Proper error handling, falls back to root if needed
• Compatible: No breaking changes to existing functionality
• Secure: Subprocess runs with appropriate user privileges