systemd: do not rely on racy heuristics to determine master PID

This fixes reloads and stops when systemd is unable to guess master process PID
due to a race described in https://trac.nginx.org/nginx/ticket/1897

While we wait for a proper fix is introduced in either nginx or systemd via
Type=forking, the workaround is not to rely on possibly unset variables, but
hardcode the path to PID file instead where we need it.