proftpd: Convert proftpd group to system scope

Proftpd is meant to be a system user, not a normal user of the
system so lets pass the correct parameters to the system.
Just add --system param when creating ftp group.

Signed-off-by: Qiang Chen <qiang.chen@windriver.com>
Signed-off-by: Jeff Polk <jeff.polk@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>