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