rc.d: Fix ftpd flags

After f99f0ee14e ("rc.d: add a service jails config to all base system
services"), the FTP service ignores the flags configured in ftpd_flags:

    # sysrc ftpd_flags=-B
    ftpd_flags:  -> -B
    # service ftpd enable
    ftpd enabled in /etc/rc.conf
    # service ftpd start
    Starting ftpd.
    # 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:	f99f0ee14e ("rc.d: add a service jails config to all base system services")
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D52745

libexec/rc/rc.d/ftpd

@@ -12,12 +12,11 @@ name="ftpd"
 desc="Internet File Transfer Protocol daemon"
 rcvar="ftpd_enable"
 command="/usr/libexec/${name}"
+command_args="-D"
 pidfile="/var/run/${name}.pid"
 
 : ${ftpd_svcj_options:="net_basic"}
 
 load_rc_config $name
 
-flags="-D ${flags} ${rc_flags}"
-
 run_rc_command "$1"