Make the setlogmask() function in the syslog module thread-safe. These changes are relevant for scenarios where the GIL is disabled or when using subinterpreters.

• The functions syslog(), openlog(), and closelog() in the syslog module are already handled for FT-Python using the @critical_section on the module. However, there might be an issue with syslog() for subinterpreters on macOS.

I will check/test the macOS subinterpreter and create a separate PR if necessary.

• Linux man page (3): setlogmast()

cc: @mpage @colesbury

• Issue: Audit all built-in modules for thread safety #116738