agent: report duplicated port in udp bsd sockets too

agent: report duplicated port in udp bsd sockets too

This patch fixes cases, where the range is full, some ports fail with
HOST_CANDIDATE_CANT_CREATE_SOCKET, other fail with
HOST_CANDIDATE_DUPLICATE_PORT, the value of res we keep when leaving the
loop is randomly the one of the last iteration of the loop.

CANT_CREATE_SOCKET still happens when trying to create an udp bsd socket
with the same address and port than one of another component, so it is
also a case of duplicate port in fact.

To be homogeneous, we add a gerror for nice_udp_bsd_socket_new(), like
we did in nice_tcp_passive_socket_new(), and we can catch the same
G_IO_ERROR_ADDRESS_IN_USE there too, when failing to get free available
udp ports.

This patch is a complement to merge request !158