Fix: main.c client/apps sockets and kernel_trace_fd close(0)

thread_registration_apps and thread_manage_clients threads are closing
sock == 0 in error handling path, which is an error.

They also may use kernel_tracer_fd at 0 value if no kernel tracer has
been found. cleanup() can close this 0 socket too, which should never
happen.

Set all these sockets as "-1" when unset.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>