Sockets: Unit tests and apps corrections

* Include lwip/inet.h in some unit tests and apps
  * Since they use htons() and pals.
* test/unit/api/test_sockets.c:
  * write() could be declared by external socket headers
    * Call lwip_write() instead.
  * Code expects fcntl() to return 6
    * But O_RDWR could have another value if external
      socket headers are present
    * Replace 6 by O_RDWR.
* apps/tftp/tftp.c:
  * recv() could be declared by external socket headers
    * Rename it to tftp_recv()

test/unit/dhcp/test_dhcp.c

@@ -4,6 +4,7 @@
 #include "lwip/dhcp.h"
 #include "lwip/prot/dhcp.h"
 #include "lwip/etharp.h"
+#include "lwip/inet.h"
 #include "netif/ethernet.h"
 
 #if LWIP_ACD