package/vsftpd: S70vsftpd: correct -x argument to start-stop-daemon

Fixes #13341

The -x / --exec start-stop-daemon option expects the path to the executable,
not just the name, leading to errors when running the init script:

Starting vsftpd: start-stop-daemon: unable to stat //vsftpd (No such file or directory)

Reported-by:  <tochansky@tochlab.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>