SimpleLink SSL support; split cert and key opts

SL requires cert and key to be separate files in DER format.

Date verification is disabled for now.

PUBLISHED_FROM=7d76150ed356140728a1e5fd82d8a0456347b7dc

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

@@ -14,8 +14,17 @@ 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 defined(MG_ENABLE_SSL)
+  #if !defined(MG_SOCKET_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 */
     mg_event_handler_t proto_handler; /* Protocol-specific event handler */