Fix subscription to wildcard topics

`mg_mqtt_match_topic_expression()` and friends are public now.

So far, only `#` is supported.

PUBLISHED_FROM=20c031bcf34be84771c4a9c456318634eb9a517c

docs/c-api/mqtt.h/intro.md

@@ -5,6 +5,7 @@ decl_name: "mqtt.h"
 items:
   - { name: mg_mqtt_connack.md }
   - { name: mg_mqtt_disconnect.md }
+  - { name: mg_mqtt_match_topic_expression.md }
   - { name: mg_mqtt_next_subscribe_topic.md }
   - { name: mg_mqtt_ping.md }
   - { name: mg_mqtt_pong.md }
@@ -17,6 +18,7 @@ items:
   - { name: mg_mqtt_subscribe.md }
   - { name: mg_mqtt_unsuback.md }
   - { name: mg_mqtt_unsubscribe.md }
+  - { name: mg_mqtt_vmatch_topic_expression.md }
   - { name: mg_send_mqtt_handshake.md }
   - { name: mg_send_mqtt_handshake_opt.md }
   - { name: mg_set_protocol_mqtt.md }

docs/c-api/mqtt.h/mg_mqtt_match_topic_expression.md

@@ -0,0 +1,12 @@
+---
+title: "mg_mqtt_match_topic_expression()"
+decl_name: "mg_mqtt_match_topic_expression"
+symbol_kind: "func"
+signature: |
+  int mg_mqtt_match_topic_expression(struct mg_str exp, struct mg_str topic);
+---
+
+Matches a topic against a topic expression
+
+Returns 1 if it matches; 0 otherwise. 
+

docs/c-api/mqtt.h/mg_mqtt_vmatch_topic_expression.md

@@ -0,0 +1,11 @@
+---
+title: "mg_mqtt_vmatch_topic_expression()"
+decl_name: "mg_mqtt_vmatch_topic_expression"
+symbol_kind: "func"
+signature: |
+  int mg_mqtt_vmatch_topic_expression(const char *exp, struct mg_str topic);
+---
+
+Same as `mg_mqtt_match_topic_expression()`, but takes `exp` as a
+NULL-terminated string. 
+