Further refine -n handling. The argument to -n is now either an absolute

Further refine -n handling.  The argument to -n is now either an absolute
path, or a path relative to $localstatedir/varnish.  By default, it is set
to the host name, which results in $localstatedir/varnish/$hostname.

This logic is centralized in instance.c, which is compiled into both
varnishd and libvarnishapi, with prototypes in varnishapi.h and heritage.h.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1581 d4fa192b-c00b-0410-8231-f00ffab90ce4