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_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 */
   
     /*