Suggestion: by default, create /run/unbound and use it for pidfile

[ronvarburg at yahoo.com]

Currently, /run/unbound.pid is the default pidfile.
I suggest to change that to /run/unbound/unbound.pid. Creating /run/unbound/ if it doesn't exists and
no other directory was configured.
Rational: to make /run tidier. It is true that unbound.pid might be the only file in /run/unbound/.
On the other hand, I think /run/unbound/ is the natural place for 
    control-interface: /run/unbound/unbound.sock
I think apache2 uses that approach. Sometimes apache2.pid is the only file under /run/apache2. Still,
it prefers /run/apache2/apache2.pid over /run/apache2.pid.
It is also true that the /run/unbound directory could be set by appropriate configuration. But having a
default setting requires less administration.