emerge --read-news: prompt only if --ask

This fixes any conditional logic that applies to --ask so that it
also applies to --read-news. For example, emerge will bail out
automatically if --ask is enabled and stdin is not a tty.

X-Gentoo-Bug: 517310
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=517310