bugfix: plug a memleak in imuxsock

The hash table for the system socket was allocated twice. The other
one being in activateListeners().

This commit practically reverts: 34a77cde2423303da72ab773128a2ddcf41
The issue seems to be that the hash table is not initialized (to NULL)
rather then not being allocated.
bugfix: plug a memleak in imuxsock

Hash table and rate limiter for the system socket weren't freed.

A side-effect was that the "Last message repeated..." notice didn't
get printed if rsyslog was shut down before the repetition was broken.
maintain ChangeLog
imuxsock: initialize system socket params only once

The system socket parameters were initialized twice and most of the
values set at first were overwritten. All the assignments are now
moved to activateListeners(). The parameters aren't accessed before
then.
imuxsock: initialize the system socket only if it's in use
imuxsock: don't try to set up sockets if there are none configured

This should be the correct version of this commit:
commit 6138ced6ce4abbaccf4c45f7b69a14811feb96ad
imuxsock: don't report an error message for $OmitLocalLogging

And in effect reverts this commit:
commit d77a8b1b8877d2dd1a0248b07564d1f88be00deb
imuxsock: When syssock.use set to off, additional listeners cannot be created
nitfix: adjust debug log message to new code path
Merge branch 'v7-stable' into v8-stable
Merge branch 'v7-stable' into v8-stable