Improve syslog facility defaults and drop prefix

The "user" facility was used for all messages; switch the default
facility for supervisor to "daemon".  Stop using the prefix strings for
stdout/stderr and instead use syslog priorities; if the user cares which
filehandle the output came on, then they can configure their syslog to
log the priority as in the below example or send it to a different file.

This makes the output look like this:

    Sep 19 10:18:46.285916 Herman daemon.notice supervisord[5296]: supervisord started with pid 5296
    Sep 19 10:18:47.288994 Herman daemon.notice supervisord[5296]: spawned: 'vmstat' with pid 5299
    Sep 19 10:18:47.291366 Herman daemon.notice supervisord[5296]: spawned: 'badvmstat' with pid 5300
    Sep 19 10:18:47.297447 Herman user.info vmstat: procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
    Sep 19 10:18:47.298812 Herman user.info vmstat:  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
    Sep 19 10:18:47.299112 Herman user.info vmstat:  3  0      0 4874284 244368 1695416    0    0    63    20   62  191  1  0 98  1
    Sep 19 10:18:47.299428 Herman user.notice badvmstat: vmstat: failed to parse argument: '5;'
    Sep 19 10:18:47.299866 Herman daemon.notice supervisord[5296]: exited: badvmstat (exit status 1; not expected)
    Sep 19 10:18:48.298243 Herman user.info vmstat:  0  0      0 4863852 244368 1702904    0    0     0     0 1437 8074  4  3 93  0
    Sep 19 10:18:48.298447 Herman daemon.notice supervisord[5296]: success: vmstat entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
    Sep 19 10:18:49.298594 Herman user.info vmstat:  0  0      0 4863528 244368 1703152    0    0     0     0  737 1475  1  1 99  0
      ...
    Sep 19 10:19:13.488894 Herman daemon.warning supervisord[5296]: received SIGINT indicating exit request
Allow syslog facility to be specified

The various 'syslog' settings now understand a syslog facility and
priority (aka severity).  Add a new type to the datatypes collection,
which lazily loads syslog if configuration options are seen.