cal: don't print terminal control characters unless stdout is a TTY

A similar change was made in svn r223931, but it was incomplete, working
only when the utility was invoked as "ncal". Fix the same issue when
invoking as "cal".

PR: 268936
Reported by: Ray Bellis <ray@bellis.me.uk>
MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D38045