Keep a master copy of the parameter block, to which all changes are applied,

Keep a master copy of the parameter block, to which all changes are applied,
and which is copied to the shared parameter block every time a parameter
changes as well as immediately before forking off a child.  This prevents a
hypothetical compromised child from changing the parent's idea of run-time
parameters (which would, for example, allow it to trick the the parent into
starting a new, hypothetically exploitable child with the attacker's choice
of uid / gid).

While I'm here, correct the use of the "volatile" qualifier - it is the
parmeter block itself which can change unpredictably, not the pointer.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1484 d4fa192b-c00b-0410-8231-f00ffab90ce4