teeworlds: do not allocate for ipv6 servers

[illwieckz]

note: it's a new PR implementing changes requested in closed #57 the major change is the master packet parser is split in two helpers for legacy and new packet formats.

• improvement: ipv6 server were just not written but an ipv4 slot was still allocated: fixed for ipv4 only lists, the server->master_pkt array is extended for each packet read for ipv4/ipv6 mixed list, the server->master_pkt array is extended for each ipv4 read
• fix: extra unused ipv4 slot for ipv6 server were mistakenly left uninitialized but readable in case of ipv6 address a slot was left uninitialized in the server->master_pkt array between two valid ipv4 address slots (even valgrind was not complaining but the bug was there)
• fix: return INPROGRESS after normal packet read too, only legacy ones was explicitely doing it
• make ipv4_header variable name more Teeworlds specific since it's a global variable
• split legacy and new packet parser in two helpers
• declare teeworlds-specific global variables as static ones
• handle allocation errors
• print debug information
• improve explanations

[stevenh]

@illwieckz did you see this failed travis build tests?

[illwieckz]

Oh wow, I was not aware it's not anymore allowed to declare a variable within the scope of a for loop… Well, I just fixed that.

[illwieckz]

thank you for the merge! :+1: