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/mg_set_ssl.md

@@ -7,12 +7,14 @@ signature: |
                          const char *ca_cert);
 ---
 
+Note: This function is deprecated, please use SSL options in mg_connect_opt.
+
 Enable SSL for a given connection.
 `cert` is a server certificate file name for a listening connection,
 or a client certificate file name for an outgoing connection.
 Certificate files must be in PEM format. Server certificate file
 must contain a certificate, concatenated with a private key, optionally
-concatenated with parameters.
+concatenated with DH parameters.
 `ca_cert` is a CA certificate, or NULL if peer verification is not
 required.
 Return: NULL on success, or error message on error. 

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

@@ -10,6 +10,9 @@ signature: |
   #ifdef MG_ENABLE_SSL
     /* SSL settings. */
     const char *ssl_cert;    /* Server certificate to present to clients */
+    const char *ssl_key;     /* Private key corresponding to the certificate.
+                                If ssl_cert is set but ssl_key is not, ssl_cert
+                                is used. */
     const char *ssl_ca_cert; /* Verify client certificates with this CA bundle */
   #endif
   };

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

@@ -10,6 +10,9 @@ signature: |
   #ifdef MG_ENABLE_SSL
     /* SSL settings. */
     const char *ssl_cert;    /* Client certificate to present to the server */
+    const char *ssl_key;     /* Private key corresponding to the certificate.
+                                If ssl_cert is set but ssl_key is not, ssl_cert
+                                is used. */
     const char *ssl_ca_cert; /* Verify server certificate using this CA bundle */
   
     /*

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 */