Refactoring toxcore/network

[kpp]

I have written implementation-independent tests so they can run both with my lib and the old one. The result of running my tests with the old toxcore is:

    Start 1: core::network
1/1 Test #1: core::network ....................***Failed    0.01 sec
Running main() from gtest_main.cc
[==========] Running 13 tests from 3 test cases.
[----------] Global test environment set-up.
[----------] 9 tests from IP
[ RUN      ] IP.from_string
[       OK ] IP.from_string (0 ms)
[ RUN      ] IP.to_string
[       OK ] IP.to_string (0 ms)
[ RUN      ] IP.ntoa
/home/humbug/2tox/tests/toxcore/test_network.cpp:128: Failure
Value of: ip_ntoa(&ip)
  Actual: "(IP invalid, family 0)"
Expected: "(IP invalid, Address family not supported by protocol)"
[  FAILED  ] IP.ntoa (0 ms)
[ RUN      ] IP.resolve
[       OK ] IP.resolve (5 ms)
[ RUN      ] IP.reset
[       OK ] IP.reset (0 ms)
[ RUN      ] IP.init
[       OK ] IP.init (0 ms)
[ RUN      ] IP.isset
[       OK ] IP.isset (0 ms)
[ RUN      ] IP.equal
[       OK ] IP.equal (0 ms)
[ RUN      ] IP.copy
[       OK ] IP.copy (0 ms)
[----------] 9 tests from IP (6 ms total)

[----------] 3 tests from IP_Port
[ RUN      ] IP_Port.isset
[       OK ] IP_Port.isset (0 ms)
[ RUN      ] IP_Port.equal
[       OK ] IP_Port.equal (0 ms)
[ RUN      ] IP_Port.copy
[       OK ] IP_Port.copy (0 ms)
[----------] 3 tests from IP_Port (0 ms total)

[----------] 1 test from Socket
[ RUN      ] Socket.valid
[       OK ] Socket.valid (0 ms)
[----------] 1 test from Socket (0 ms total)

[----------] Global test environment tear-down
[==========] 13 tests from 3 test cases ran. (6 ms total)
[  PASSED  ] 12 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] IP.ntoa

 1 FAILED TEST
  YOU HAVE 3 DISABLED TESTS



0% tests passed, 1 tests failed out of 1

Total Test time (real) =   0.01 sec

The following tests FAILED:
      1 - core::network (Failed)

1 fail is a little improvement of error messaging. The other 3 are segfaults caused by incorrect handling of pointers(they are not checked against null in the old implementation).