WMNut-0.72 with revised CI recipes and hardened code base

• Update GitHub Actions recipes to test both GCC and CLANG, use CCACHE to speed up the phase of custom-built NUT [#23]
• Revise configure.ac for better portability (or detection of incapable build environments); introduce wmnut-common.h to handle platform specifics (like NUT_PATH_MAX) once [#20]
• Code-hardening (address static analysis warnings, suggestions from GitHub code scanning AI bots, manual review) [#17, #21, #22]
• Safer use of configuration file path relative to the user home directory [#18]
• Fix conflicts of global and method-local variable names, rename some of the variables and methods to better match their purpose [#16, #19]