Increase LWIP MEM_SIZE to > TCP_SND_BUF (#731)

The send buffers are set to 8 * MSS = ~11.5K.  MEM_SIZE is now set to
be larger than that, 16K, in order to help avoid having tcp_write fail
with ENOMEM.  The attempt to use a smaller size is still included, which
will allow a tcp_write of up to 16 * 16K = 256K, i.e. all of memory.

Fixes #725

include/lwipopts.h

@@ -21,7 +21,7 @@ extern void interrupts();
 #define MEM_LIBC_MALLOC             0
 
 #define MEM_ALIGNMENT               4
-#define MEM_SIZE                    8192
+#define MEM_SIZE                    16384
 #define MEMP_NUM_TCP_SEG            32
 #define MEMP_NUM_ARP_QUEUE          10
 #define PBUF_POOL_SIZE              24

tools/libpico/lwipopts.h

@@ -21,7 +21,7 @@ extern void interrupts();
 #define MEM_LIBC_MALLOC             0
 
 #define MEM_ALIGNMENT               4
-#define MEM_SIZE                    8192
+#define MEM_SIZE                    16384
 #define MEMP_NUM_TCP_SEG            32
 #define MEMP_NUM_ARP_QUEUE          10
 #define PBUF_POOL_SIZE              24