C Clubby refactoring

Complete rewrite of the C Clubby implementation: it is now modeled after
the Go implementation, with a slight difference that codec and channel
are the same object (something we should probably do in Go as well, as
we only have a single type of channel so far, which is used with all
types of codecs).

This implementation also comes with a new external API,
which is hopefully cleaner and easier to use (see mg_clubby.h).

In this PR I am not adding any new types of channels, but a UART channel
as well as websocket listener channel will be added after this lands.

PUBLISHED_FROM=d545d4bb6434e2a02ad159f9e4b64e594a4797e7

mongoose.h

@@ -811,6 +811,9 @@ int mg_vcmp(const struct mg_str *str2, const char *str1);
  */
 int mg_vcasecmp(const struct mg_str *str2, const char *str1);
 
+struct mg_str mg_strdup(const struct mg_str s);
+int mg_strcmp(const struct mg_str str1, const struct mg_str str2);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */