proftpd: move pidfile from /var/ to /var/run/

/var/run/ is more suitable to store pidfile for OE, and this fixes
"/etc/init.d/proftpd stop" failure too, since this script assumes
the pidfile is under /var/run

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>