Remove .listener member of controlistener struct

Remove .listener member of controlistener struct

In the new netmgr code, the .listener member was mostly functionally
only duplicating the .exiting member and was unneeded.  This also
resolves following ThreadSanitizer (harmless) warning:

    WARNING: ThreadSanitizer: data race
      Write of size 1 at 0x000000000001 by thread T1:
	#0 control_senddone bin/named/controlconf.c:257:22
	#1 tcp_send_cb lib/isc/netmgr/tcp.c:1027:2
	#2 uv__write_callbacks /home/ondrej/Projects/tsan/libuv/src/unix/stream.c:953:7
	#3 uv__stream_io /home/ondrej/Projects/tsan/libuv/src/unix/stream.c:1330:5
	#4 uv__run_pending /home/ondrej/Projects/tsan/libuv/src/unix/core.c:812:5
	#5 uv_run /home/ondrej/Projects/tsan/libuv/src/unix/core.c:377:19
	#6 nm_thread lib/isc/netmgr/netmgr.c:500:11

      Previous write of size 1 at 0x000000000001 by thread T2:
	#0 control_senddone bin/named/controlconf.c:257:22
	#1 tcp_send_cb lib/isc/netmgr/tcp.c:1027:2
	#2 uv__write_callbacks /home/ondrej/Projects/tsan/libuv/src/unix/stream.c:953:7
	#3 uv__stream_io /home/ondrej/Projects/tsan/libuv/src/unix/stream.c:1330:5
	#4 uv__run_pending /home/ondrej/Projects/tsan/libuv/src/unix/core.c:812:5
	#5 uv_run /home/ondrej/Projects/tsan/libuv/src/unix/core.c:377:19
	#6 nm_thread lib/isc/netmgr/netmgr.c:500:11

      Location is heap block of size 265 at 0x000000000009 allocated by thread T3:
	#0 malloc <null>
	#1 default_memalloc lib/isc/mem.c:713:8
	#2 mem_get lib/isc/mem.c:622:8
	#3 isc___mem_get lib/isc/mem.c:1044:9
	#4 isc__mem_get lib/isc/mem.c:2432:10
	#5 add_listener bin/named/controlconf.c:1133:13
	#6 named_controls_configure bin/named/controlconf.c:1331:6
	#7 load_configuration bin/named/server.c:9133:2
	#8 run_server bin/named/server.c:9771:2
	#9 dispatch lib/isc/task.c:1152:7
	#10 run lib/isc/task.c:1344:2

      Thread T2 (running) created by main thread at:
	#0 pthread_create <null>
	#1 isc_thread_create lib/isc/pthreads/thread.c:73:8
	#2 isc_nm_start lib/isc/netmgr/netmgr.c:223:3
	#3 create_managers bin/named/main.c:909:15
	#4 setup bin/named/main.c:1223:11
	#5 main bin/named/main.c:1523:2

      Thread T2 (running) created by main thread at:
	#0 pthread_create <null>
	#1 isc_thread_create lib/isc/pthreads/thread.c:73:8
	#2 isc_nm_start lib/isc/netmgr/netmgr.c:223:3
	#3 create_managers bin/named/main.c:909:15
	#4 setup bin/named/main.c:1223:11
	#5 main bin/named/main.c:1523:2

      Thread T3 (running) created by main thread at:
	#0 pthread_create <null>
	#1 isc_thread_create lib/isc/pthreads/thread.c:73:8
	#2 isc_taskmgr_create lib/isc/task.c:1434:3
	#3 create_managers bin/named/main.c:915:11
	#4 setup bin/named/main.c:1223:11
	#5 main bin/named/main.c:1523:2

    SUMMARY: ThreadSanitizer: data race bin/named/controlconf.c:257:22 in control_senddone