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*’

Note that I'm not sure if this is complete. There is also `char * parsed` in
`struct cfg_defvalue_t`, and I guess that should probably be `const` too. But
I don't know for sure because I'm not using that functionality in libconfuse.