tracing of active sockets and handles

tracing of active sockets and handles

If NETMGR_TRACE is defined, we now maintain a list of active sockets
in the netmgr object and a list of active handles in each socket
object; by walking the list and printing `backtrace` in a debugger
we can see where they were created, to assist in in debugging of
reference counting errors.

On shutdown, if netmgr finds there are still active sockets after
waiting, isc__nm_dump_active() will be called to log the list of
active sockets and their underlying handles, along with some details
about them.

(cherry picked from commit 00e04a86c8b6828a066573031cc539adab565061)