Refactor mg polling

* Change return type of mg_mgr_poll to return number of events
 * Add mg_mgr_min_timer
 * Refactor main poll loop to remove LwIP-specific stuff

CL: Refactor mg polling

PUBLISHED_FROM=dc94618b32fa3c84a2f053bd04d134297780ec82

docs/c-api/mg_net.h/mg_mgr_free.md

@@ -3,7 +3,7 @@ title: "mg_mgr_free()"
 decl_name: "mg_mgr_free"
 symbol_kind: "func"
 signature: |
-  void mg_mgr_free(struct mg_mgr *);
+  void mg_mgr_free(struct mg_mgr *mgr);
 ---
 
 De-initialises Mongoose manager.

docs/c-api/mg_net.h/mg_mgr_poll.md

@@ -3,11 +3,11 @@ title: "mg_mgr_poll()"
 decl_name: "mg_mgr_poll"
 symbol_kind: "func"
 signature: |
-  time_t mg_mgr_poll(struct mg_mgr *, int milli);
+  int mg_mgr_poll(struct mg_mgr *mgr, int milli);
 ---
 
 This function performs the actual IO and must be called in a loop
-(an event loop). It returns the current timestamp.
+(an event loop). It returns number of user events generated (except POLLs).
 `milli` is the maximum number of milliseconds to sleep.
 `mg_mgr_poll()` checks all connections for IO readiness. If at least one
 of the connections is IO-ready, `mg_mgr_poll()` triggers the respective

docs/c-api/mg_net.h/struct_mg_mgr.md

@@ -13,6 +13,7 @@ signature: |
   #endif
     void *user_data; /* User data */
     int num_ifaces;
+    int num_calls;
     struct mg_iface **ifaces; /* network interfaces */
     const char *nameserver;   /* DNS server to use */
   };