Pull out common SSL interface

Currently only OpenSSL API impl, but the plan is to add more.

PUBLISHED_FROM=c85b26fa2312047768bdcf3eca583e60862aba90

docs/c-api/net.h/struct_mg_connection.md

@@ -14,19 +14,11 @@ signature: |
     size_t recv_mbuf_limit;  /* Max size of recv buffer */
     struct mbuf recv_mbuf;   /* Received data */
     struct mbuf send_mbuf;   /* Data scheduled for sending */
-  #if MG_ENABLE_SSL
-  #if MG_NET_IF != MG_NET_IF_SIMPLELINK
-    SSL *ssl;
-    SSL_CTX *ssl_ctx;
-  #else
-    char *ssl_cert;
-    char *ssl_key;
-    char *ssl_ca_cert;
-    char *ssl_server_name;
-  #endif
-  #endif
     time_t last_io_time;              /* Timestamp of the last socket IO */
     double ev_timer_time;             /* Timestamp of the future MG_EV_TIMER */
+  #if MG_ENABLE_SSL
+    void *ssl_if_data; /* SSL library data. */
+  #endif
     mg_event_handler_t proto_handler; /* Protocol-specific event handler */
     void *proto_data;                 /* Protocol-specific data */
     void (*proto_data_destructor)(void *proto_data);