fix: add freenginx compatibility for request timing calculation

[u5surf]

Summary

• Add conditional compilation to handle different request timing fields between nginx and freenginx
• For freenginx: use r->start_time (unified millisecond timestamp)
• For nginx: use r->start_sec and r->start_msec (separate fields)
• Fixes compilation error: 'ngx_http_request_t' has no member named 'start_msec'

Problem

The VTS module failed to build with freenginx because it used r->start_msec which doesn't exist in freenginx. Freenginx has consolidated the timing fields into a single start_time field.

Solution

Added conditional compilation to handle both nginx variants:

#ifdef FREENGINX_VERSION
    ms = (ngx_msec_int_t)(tp->sec * 1000 + tp->msec - r->start_time);
#else
    ms = (ngx_msec_int_t)
             ((tp->sec - r->start_sec) * 1000 + (tp->msec - r->start_msec));
#endif

Test plan

• Successfully builds with standard nginx (tested)
• Test with freenginx build
• Verify timing calculations work correctly in both environments
• Run existing test suite

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR adds compatibility support for the freenginx web server by implementing conditional compilation to handle different request timing field structures between nginx and freenginx. The change resolves a compilation error where freenginx doesn't have the start_msec field that nginx uses.

• Adds conditional compilation using FREENGINX_VERSION macro to differentiate between nginx and freenginx builds
• Implements different timing calculation logic for each server variant
• Maintains backward compatibility with existing nginx installations