Fix port leaking after controlling_process(Port, self())

Add case to handle the situation when someone call
{gen_tcp,gen_udp}:controlling_process(Port, self()). Also improve spec
and doc from gen_udp and gen_sctp for controlling_process/2.

To reproduce the issue, open an UDP port:

4> {ok,Port} = gen_udp:open(9000, [binary]).
{ok,#Port<0.587>}
5> gen_udp:controlling_process(Port, self()).
ok

Simulate error:

6> 1=2.
** exception error: no match of right hand side value 2

Here is the leak:

7> inet:i().
Port Module   Recv Sent Owner    Local Address Foreign Address State
Type
581  inet_udp 0    0    <0.31.0> *:cslistener  *:*             BOUND
DGRAM
ok

1 files changed, 3 insertions, 2 deletions

diff --git a/lib/kernel/src/gen_udp.erl b/lib/kernel/src/gen_udp.erl
index 8688799ae9..914854c65c 100644
--- a/lib/kernel/src/gen_udp.erl
+++ b/lib/kernel/src/gen_udp.erl
@@ -185,9 +185,10 @@ connect(S, Address, Port) when is_port(S) ->
 	    Error
     end.
 
--spec controlling_process(Socket, Pid) -> ok when
+-spec controlling_process(Socket, Pid) -> ok | {error, Reason} when
       Socket :: socket(),
-      Pid :: pid().
+      Pid :: pid(),
+      Reason :: closed | not_owner | inet:posix().
 
 controlling_process(S, NewOwner) ->
     inet:udp_controlling_process(S, NewOwner).