After f99f0ee14e3a ("rc.d: add a service jails config to all base system
services"), the FTP service ignores the flags configured in ftpd_flags:

1. sysrc ftpd_flags=-B ftpd_flags: -> -B
2. service ftpd enable ftpd enabled in /etc/rc.conf
3. service ftpd start Starting ftpd.
4. pgrep -fl ftpd 1234 /usr/libexec/ftpd -D

Notice the absence of the "-B" flag.

PR: 285600
Fixes: f99f0ee14e3a ("rc.d: add a service jails config to all base system services")