rc.d: Fix ftpd flags

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
Reviewed by: 0mp, emaste
Approved by: emaste (mentor)
Fixes: f99f0ee14e3a ("rc.d: add a service jails config to all base system services")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52745

(cherry picked from commit 2872ced18742aaf9480c9a43059cba7e24dd7e59)