net: conn: Check duplicate UDP/TCP connection handlers

When a new UDP or TCP connection handler is to be registered,
we need to check if identical handler has already been created.
If a duplicate is found, the registering call will return -EALREADY.

The earlier code did not check this but allowed two identical
handlers to be created. The latter handler was never called in
this case.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>