Allow the callers of `CreateSock()` to pass all 3 arguments to the
`socket(2)` syscall. This makes it possible to create sockets of
any domain/type/protocol.
This is an extraction of ArgsManager related functions from util/system
into their own common file.
Config file related functions are moved to common/config.cpp.
The background of this commit is an ongoing effort to decouple the
libbitcoinkernel library from the ArgsManager. The ArgsManager belongs
into the common library, since the kernel library should not depend on
it. See doc/design/libraries.md for more information on this rationale.
