imjournal: Don't drop messages without the MESSAGE field
imjournal: Set default severity for all messages

Some messages comming from journald don't have the SYSLOG_PRIORITY
field. These are typically the messages logged through journald's
native API. Set the default severity for these messages to 'notice'.
imjournal: Set default facility for all messages

Some messages comming from journald don't have the SYSLOG_FACILITY
field. These are typically the messages logged through journald's
native API. Set the default facility for these messages to 'user'.
imjournal: Make default priority configurable
maintain ChangeLog