Use hardlinks for active_vcl instead of symlink

This solves the following scenario:

Write vcl, store it as foo (tmp/foo.vcl).
Deploy vcl, tmp/active_vcl points to tmp/foo.vcl
Restart varnish, foo is now a legal name again
Write bogus but syntactically correct vcl, store it as foo (tmp/foo.vcl)
tmp/active_vcl now points to a vcl that isn't deployed and never has been.

Still some issues remaining, but this is decent.