Fix missing return statement ssl_server2 idling

Also, introduce MBEDTLS_EINTR locally in net_sockets.c
for the platform-dependent return code macro used by
the `select` call to indicate that the poll was interrupted
by a signal handler: On Unix, the corresponding macro is EINTR,
while on Windows, it's WSAEINTR.

programs/ssl/ssl_client2.c

@@ -462,7 +462,7 @@ int idle( mbedtls_net_context *fd,
         poll_type = MBEDTLS_NET_POLL_READ;
 #if !defined(MBEDTLS_TIMING_C)
     else
-        return;
+        return( 0 );
 #endif
 
     while( 1 )

programs/ssl/ssl_server2.c

@@ -863,7 +863,7 @@ int idle( mbedtls_net_context *fd,
         poll_type = MBEDTLS_NET_POLL_READ;
 #if !defined(MBEDTLS_TIMING_C)
     else
-        return;
+        return( 0 );
 #endif
 
     while( 1 )