Change manager to use mgt_event.h instead of threads to be lazy thread

Change manager to use mgt_event.h instead of threads to be lazy thread
developer compatible.

POSIX, no surprise, doesn't really tell what should happen to a threaded
process which forks and consequently implemenations vary somewhat,
from Solaris which seems to Do The Right Thing, via Linux where it
works "most of the time" and to FreeBSD which more or less actively
sabotages any such attempt.

Grin and live with it...

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@653 d4fa192b-c00b-0410-8231-f00ffab90ce4