Introduce MG_NET_IF var, replaces MG_NET_IF_*

Possible values:
  MG_NET_IF_SOCKET (1) - traditional BSD socket API
  MG_NET_IF_SIMPLELINK (2) - TI's SimpleLink socket interface
  MG_NET_IF_LWIP_LOW_LEVEL - a net_if implementation that uses LWIP's
    low-level API, when LWIP_SOCKET is not available.

PUBLISHED_FROM=a5c0aebb7b6bdb1f1f4cb3210763d16f3ab5e133

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

@@ -15,7 +15,7 @@ signature: |
     struct mbuf recv_mbuf;   /* Received data */
     struct mbuf send_mbuf;   /* Data scheduled for sending */
   #if MG_ENABLE_SSL
-  #if !defined(MG_SOCKET_SIMPLELINK)
+  #if MG_NET_IF != MG_NET_IF_SIMPLELINK
     SSL *ssl;
     SSL_CTX *ssl_ctx;
   #else