bugfix: prevent another endless loop in the ratelimiter

The message that reports how many messages were lost due to
ratelimiting was sent before reseting the state that led to it. If it
itself got ratelimited, this could lead to an endless loop.
bugfix: prevent calling tellLostCnt() twice
Fix rsyslog.service to comply with systemd recommendations

Those are documented here:

http://www.freedesktop.org/wiki/Software/systemd/syslog/
systemd: use service type notify

There is a time window, between rsyslog reporting syntax
errors and the daemon returning with failure, this may cause

systemctl restart rsyslog to not report any error inmediately
but later in the logs which is confusing to users.

The appropiate steps to correct this annoyance is to notify
systemd with a simple sd_notify(0, "READY=1"); just before
entering the main loop.

Tested in openSUSE 12.3/13.1 x86_64
bugfix: prevent a segfault if state file is not defined
doc: mention contributed patch in ChangeLog
doc: warn about using imjournal

do to bugs in systemd, the module can lead to a DoS to the local machine
Merge branch 'v7-stable'