Migrate from PCRE to PCRE2 API

[Copilot]

This PR updates the xrootd-cmstfc repository to support PCRE2, replacing the legacy PCRE library with the modern PCRE2 API. This migration ensures compatibility with modern systems where legacy PCRE is deprecated or unavailable.

Changes Made

CMake Configuration

• Added new FindPcre2.cmake module to locate PCRE2 library and headers
• Updated CMakeLists.txt to use PCRE2 instead of PCRE
• Removed obsolete FindPcre.cmake configuration

Source Code Migration

• Updated header includes from <pcre.h> to <pcre2.h> with proper code unit width definition
• Migrated data types from pcre * to pcre2_code *
• Replaced PCRE function calls with PCRE2 equivalents:
  • pcre_compile() → pcre2_compile()
  • pcre_exec() → pcre2_match()
  • pcre_free() → pcre2_code_free() and pcre2_match_data_free()
• Updated error handling to use PCRE2's error number/offset pattern
• Implemented PCRE2 match data structures for proper memory management

API Differences Handled

• PCRE2 requires explicit code unit width definition (PCRE2_CODE_UNIT_WIDTH 8)
• Match results now use pcre2_match_data structures instead of raw arrays
• Updated constant names (e.g., PCRE_ANCHORED → PCRE2_ANCHORED)
• Proper cleanup of match data structures to prevent memory leaks

Verification

• Clean build completes successfully with PCRE2
• Library properly links to libpcre2-8
• Module loads correctly and exports expected symbols
• Maintains backward compatibility with existing XRootD plugin interface

This migration ensures the project remains maintainable and compatible with modern Linux distributions that are phasing out legacy PCRE in favor of PCRE2.

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