Verify that port numbers in cfg are unique

memcached uses the port number of an interface to uniquely identify
that interface. If a user attempts to configure two interfaces on the
same port (including port 0 for 'any available') then memcached's
connection management quickly breaks.

Therefore add a check that the port for an interface is unique.

Change-Id: I70b6ce0ea1a98e37d794de75b9b96b0f6d720cbe
Reviewed-on: http://review.couchbase.org/42714
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>