net: context: Add helper function for checking socket registration

We need to know if we have registered a socket listener, otherwise
we try to register UDP handler too many times.

Change-Id: I060150d01eb149f2a2a6a332d18e0142e5cb7a5e
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>