Make some char * into const char * to avoid C++ compiler warnings.

This avoids C++ compiler warnings when using libconfuse in C++ programs
and providing a string literal to CFG_zzz macros. It affects config item
names, and also CFG_STR default values.
    CFG_STR("log-config", "log.conf", CFGF_NONE),
warning: deprecated conversion from string constant to ‘char*’