debug GUI: make sure logfile FILE* pointer NULL. cleanup code related to
opening a logfile.
remove GFX_ShowMsg(), pass all logging through DEBUG_ShowMsg(). Remove
conditional mapping in logging.h, LOG_MSG is now always DEBUG_ShowMsg().
Fix debug GUI code so that non-debug and debug builds always have the
[log] section and logging function. Update non-debug DEBUG_ShowMsg() to
print the message to stderr if a logfile is not provided.