configure.ac: use /run/nagios.lock as the default lockfile path.

The previous default path for the lockfile was located in
$localstatedir, which is generally writable by the nagios user. That
presents a security risk, since the nagios user can delete the
lockfile and replace it with another file containing the PID of a
root-owned process. This commit changes the default lockfile path to
/run/nagios.lock, and completes the fix for Github issue #404.