rc.d/ntp: Ensure ntpd.leap-seconds.list is readable by ntpd

When a use sets umask in login.conf(5) to 027 or 077 a subsequently
fetched /var/db/ntpd.leap-seconds.list will inherit the permissions
allowed by the umask, resulting in a file that may not be readable
ntpd running under the ntp account. This patch adds a umask command
to preempt the umask in login.conf(5) prior to fetching a new copy
of the leap-seconds file.

PR: 261298
Reported by: Martin Waschbusch <martin@waschbuesch.de>
MFC after: 3 days