Make some logs less noisy (#495)

• log: lower IPC connection issues to info level

... in handle_new_connection(). The caller has better context for whether a
problem merits a warning or error, and the function's return code is
sufficiently descriptive to do so. Some problems may be expected or able to be
worked around.

For example, Pacemaker's crm_mon attempts to contact pacemakerd IPC. On a
Pacemaker Remote node, that IPC will be unavailable, and crm_mon can check the
libqb return code to detect and handle that situation gracefully.

• log: lower some ringbuffer debug messages to trace level

They're rather noisy, with every shm-based IPC connection generating multiple
obscure messages like:

  debug: shm size:1048589; real_size:1052672; rb->word_size:263168

and every disconnect generating the rather unhelpful:

  debug: qb_ipcc_disconnect()

along with multiple messages like:

  debug: Closing ringbuffer: /dev/shm/qb-10986-11014-34-26VRvs/qb-request-cmap-header

All of these seem appropriate to trace level.