Cleanup hton*/ntoh* function handling and platform abstraction

Let lwip use functions/macros prefixed by lwip_ internally to avoid naming clashes with external #includes.
Remove over-complicated #define handling in def.h
Make functions easier to override in cc.h. The following is sufficient now (no more LWIP_PLATFORM_BYTESWAP):
#define lwip_htons(x) <your_htons>
#define lwip_htonl(x) <your_htonl>

doc/rawapi.txt

@@ -456,9 +456,8 @@ introduction to this subject.
 Other significant improvements can be made by supplying
 assembly or inline replacements for htons() and htonl()
 if you're using a little-endian architecture.
-#define LWIP_PLATFORM_BYTESWAP 1
-#define LWIP_PLATFORM_HTONS(x) <your_htons>
-#define LWIP_PLATFORM_HTONL(x) <your_htonl>
+#define lwip_htons(x) <your_htons>
+#define lwip_htonl(x) <your_htonl>
 
 Check your network interface driver if it reads at
 a higher speed than the maximum wire-speed. If the