configure: don't use fallback values in summary

They are not needed anymore, if default values are used, they are expanded.
configure: use `runstatedir` instead of /var/run in configure
configure: make default socket location configurable in configure

This closes #63.
configure: make default PID location configurable

Also, document those changes in NEWS as using `localstatedir` may move
stuff from `/var` to `/usr/local/var`.

Closes #64.
man: use configured path for /var/run/lldpd.socket