This function has been causing somewhat rare crashes (e.g. one per week on a very active system with MQTT enabled) on both Windows and Linux (see issue #495). The root-cause is still unclear (possible heap corruption?). This function needed more robustification anyway (see previous TODO comment), so I'm refactoring here to no longer use strListAppendFormat, which uses vasprintf (the function where the rare crash was occurring) and instead just use snprintf and strListPush. The total client_list that's published (as a single string/message) is now dynamically allocated (this was the point of the previous TODO comment). This may not actually fix the issue if there's a heap corruption occurring somewhere else in this function or the call-chain that's reaching here (usually from the web server).

Another change to mqtt_client_on(): don't incremented the server's 'served' counter until client disconnects (this is a past-tense statistic).

Also:
- mqtt_pub_strval() can now accept a null 'str' argument which is effectively the same as mqtt_pub_noval(), so the pub_noval() function is now just a thin wrapper around pub_strval().

- mqtt_startup() now clears the client_list topic and sets the client_count topic to 0.