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.