Fork domain socket support out of //net

The code now in mojo/shell/domain_socket is forked out of net/socket,
and some out of net/base. I removed error codes that aren't relevant
to Unix Domain Sockets, and also all the code that allows reading
and writing over these sockets. We only need to listen, connect, and
accept on domain sockets.

BUG=425711
TEST=external_application_tests
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/678583002