imjournal: make $IgnorePreviousMessages take effect even without a state file

Previously, $StateFile had to be specified for $IgnorePreviousMessages
to work.
Correct and slightly extend the man page section on debugging
Amend the rsyslog.conf man page with the file sync syntax
bugfix: testbench failed if imptcp was not enabled
tests: imdiag dependency is implied by testbench
Merge pull request #163 from rsyslog/master-th-tests-cleanup

Two small changes to tests
imjournal: add a new option UsePIDFromSystem

This option is analogous to the option of the same name from imuxsock.
When set, the PID is taken from the journal rather than the message.

Conflicts:
	plugins/imjournal/imjournal.c
fix datatype conversion