functions for converting between host and network byte order. These
names are the de facto standard names for this functionality because
of the original BSD TCP/IP implementation. But they cause problems for
uIP/Contiki: some platforms define these names themselves (Mac OS,
most notably), causing compilation problems for Contiki on those
platforms.

This commit changes all htons to uip_htons instead. Same goes for
htonl, ntohs, and ntohl. All-caps versions as well.