Add IPv6 configuration option

Disabled in the default config file, but enabled by default if not specified,
for backward compatibility.

src/conffile.c

@@ -47,6 +47,7 @@ static cfg_opt_t sec_general[] =
     CFG_STR("logfile", STATEDIR "/log/" PACKAGE ".log", CFGF_NONE),
     CFG_STR("db_path", STATEDIR "/cache/" PACKAGE "/songs3.db", CFGF_NONE),
     CFG_INT_CB("loglevel", E_LOG, CFGF_NONE, &cb_loglevel),
+    CFG_BOOL("ipv6", cfg_true, CFGF_NONE),
     CFG_END()
   };