Rename error types and constants.

We'll be using them in layers which aren't really Profinet, so PROFINET_
didn't make a lot of sense.
Plus, this is shorter to type.
Use gnu99 rather that C99.

Unfortunately pcap isn't C99 compliant, leading to compilation problems.
See http://stackoverflow.com/questions/15393905/c-pcap-library-unknown-types-error
ppkt: Allow ppkt_t to be created with external storage.

ppkt_alloc() allocates a new ppkt_t with internal storage,
ppkt_create() creates a new ppkt_t with external storage.
This is useful for the pcap implementation, so we don't have to copy
data into a ppkt_t, we just create a ppkt_t wrapper around the buffer
pcap gives us.