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, as well as any processes which do not have
a logfile explicitly set.
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.
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.