This eliminates read-add-zero-write on counters outside this group.

in VSC_C_main and protect them with the ban-mtx.

reports stuff from it for debugging.

size_t is 64 bit.

Fixes: #2780

Varnish Cache 6.1.0 has been released, and we are moving
back to trunk. A 6.1 branch was created from the preivious
"Prepare for.." commit.

I consider it too late for a vtc, but this has been tested manually
with ulimit -n 40. Example errors (tcp, uds)

         0 SessError      - a0 127.0.0.1 8080 3 24 Too many open files
	 0 SessError      - a1 0.0.0.0 0 5 24 Too many open files

Fixes: #2777

facility.