dnsmasq: fix remove pidfile on shutdown regression

Regression introduced by 3481d0d7 dnsmasq: run as dedicated UID/GID

dnsmasq is unable to remove its own pidfile as /var/run/dnsmasq is owned
by root and now dnsmasq runs as dnsmasq:dnsmasq.  Change directory
ownership to match.

dnsmasq initially starts as root, creates the pidfile, then drops to
requested non-root user.  Until this fix dnsmasq had insufficient
privilege to remove its own pidfile.

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>