Make syslog configurable again

Setting the logfile to 'syslog' did not work because it was treated as a
path and normalized to '/syslog', thwarting a later branch.  Allow it to
work again, and add a new 'syslog' configuration parameter which makes the
main program output to syslog.
Use syslog tags for output from child processes

The output from child processes was not including the channel; use the
syslog tag for this purpose.  Indicate the channel via a format prefix.
Extend the handle_syslog function to take these new parameters, and make
the callers call them.

Before:

    Sep 18 22:58:48 Herman supervisord: 2013-09-18 22:58:48,828 CRIT Supervisor running as root (no user in config file)
    Sep 18 22:58:48 Herman supervisord: 2013-09-18 22:58:48,839 INFO daemonizing the supervisord process
    Sep 18 22:58:48 Herman supervisord: 2013-09-18 22:58:48,840 INFO supervisord started with pid 12196
    Sep 18 22:58:49 Herman supervisord: 2013-09-18 22:58:49,842 INFO spawned: 'vmstat' with pid 12207
    Sep 18 22:58:49 Herman supervisord: vmstat procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
    Sep 18 22:58:49 Herman supervisord: vmstat  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
    Sep 18 22:58:49 Herman supervisord: vmstat  1  0      0 4092856 260072 2152232    0    0    34    28  138  538  3  1 96  0
    Sep 18 22:58:50 Herman supervisord: vmstat  0  0      0 4092972 260080 2152224    0    0     0   176  669 2658  1  1 98  0
    Sep 18 22:58:50 Herman supervisord: 2013-09-18 22:58:50,851 INFO success: vmstat entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
    Sep 18 22:58:51 Herman supervisord: vmstat  0  0      0 4093168 260080 2152272    0    0     0     8  584 1888  1  1 99  0
    Sep 18 22:58:52 Herman supervisord: vmstat  0  0      0 4092920 260080 2152272    0    0     0     0  639 2225  1  0 98  0
    Sep 18 22:58:53 Herman supervisord: vmstat  0  0      0 4092468 260080 2152312    0    0     0     0  690 2527  1  1 98  0
    Sep 18 22:58:53 Herman supervisord: 2013-09-18 22:58:53,852 INFO exited: vmstat (exit status 0; expected)

After:

    Sep 18 22:59:15 Herman supervisord[11556]: Supervisor running as root (no user in config file)
    Sep 18 22:59:15 Herman supervisord[11559]: daemonizing the supervisord process
    Sep 18 22:59:15 Herman supervisord[11559]: supervisord started with pid 11559
    Sep 18 22:59:16 Herman supervisord[11559]: spawned: 'vmstat' with pid 11570
    Sep 18 22:59:16 Herman vmstat: stdout: procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
    Sep 18 22:59:16 Herman vmstat: stdout:  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
    Sep 18 22:59:16 Herman vmstat: stdout:  1  0      0 4115832 259304 2142632    0    0    38    30  143  562  3  1 96  0
    Sep 18 22:59:17 Herman vmstat: stdout:  0  0      0 4116600 259304 2141304    0    0     0     0  796 5189  2  1 97  0
    Sep 18 22:59:17 Herman supervisord[11559]: success: vmstat entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
    Sep 18 22:59:18 Herman vmstat: stdout:  0  0      0 4116684 259312 2141260    0    0     0   160  646 2230  2  0 98  0
    Sep 18 22:59:19 Herman vmstat: stdout:  1  0      0 4117064 259312 2141260    0    0     0     0  737 3086  2  1 98  0
    Sep 18 22:59:20 Herman vmstat: stdout:  1  0      0 4116836 259312 2141260    0    0     0   204  583 1677  1  0 99  0
    Sep 18 22:59:20 Herman supervisord[11559]: exited: vmstat (exit status 0; expected)

There is no priority visible in this example but that is syslog for you.
Make syslog handler pass priority of events through

Previously the level of the alert was only formatted by the non-syslog
handlers; pass it through as the syslog priority.

Now our syslog stream looks like this:

    Sep 18 23:39:03.588720 Herman user.warning supervisord[18161]: Failed to clean up '/tmp/vmstat-stderr---supervisor-Q3tGtz.log'
    Sep 18 23:39:03.600233 Herman user.notice supervisord[18164]: daemonizing the supervisord process
    Sep 18 23:39:03.600682 Herman user.notice supervisord[18164]: supervisord started with pid 18164
    Sep 18 23:39:04.596211 Herman user.notice supervisord[18164]: spawned: 'vmstat' with pid 18175
    Sep 18 23:39:04.603711 Herman user.notice vmstat: stdout: procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
    Sep 18 23:39:04.604265 Herman user.notice vmstat: stdout:  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
    Sep 18 23:39:04.604553 Herman user.notice vmstat: stdout:  3  0      0 3979464 264004 2221996    0    0    24    21  119  470  2  1 97  0
    Sep 18 23:39:05.604561 Herman user.notice vmstat: stdout:  0  0      0 3986896 264004 2214836    0    0     0     0  612 3942  1  0 99  0
    Sep 18 23:39:05.605355 Herman user.notice supervisord[18164]: success: vmstat entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
    Sep 18 23:39:08.605574 Herman user.notice vmstat: stdout:  0  0      0 3985940 264008 2215212    0    0     0     8  598 2064  1  0 99  0
    Sep 18 23:39:08.606140 Herman user.notice supervisord[18164]: exited: vmstat (exit status 0; expected)
    Sep 18 23:39:37.159440 Herman user.warning supervisord[18164]: received SIGTERM indicating exit request
Document and changelog syslog enhancements