`Sock::Get()` was used only in `sock.{cpp,h}`. Remove it and access
`Sock::m_socket` directly.
Unit tests that used `Get()` to test for equality still verify that the
behavior is correct by using the added `operator==()`.
Since the kernel library no longer depends on the system file, move it
to the common library instead in accordance to the diagram in
doc/design/libraries.md.
Outside of `Sock`, `Sock::Reset()` was used in just one place (in
`i2p.cpp`) which can use the assignment operator instead.
This simplifies the public `Sock` API by having one method less.
This fixes:
```bash
In file included from test/sock_tests.cpp:10:
In file included from /usr/local/include/boost/test/unit_test.hpp:18:
In file included from /usr/local/include/boost/test/test_tools.hpp:46:
/usr/local/include/boost/test/tools/old/impl.hpp:107:17: warning: comparison of integers of different signs: 'const long' and 'const unsigned long' [-Wsign-compare]
return left == right;
~~~~ ^ ~~~~~
```
which was introduced in #20788.
