Include all pre-generated man pages in "make dist"

Some man pages (e.g. dnstap-read.1, named-nzd2nzf.1) should only be installed conditionally (when the relevant features are enabled in a given BIND 9 build). This is achieved using Automake conditionals. However, while all source reStructuredText files are included in tarballs produced by "make dist" (distribution tarballs) as they should be, the list of pre-generated man pages included in distribution tarballs incorrectly depends on the ./configure switches used for the build which "make dist" is run for. Meanwhile, distribution tarballs should always contain all the files necessary to build any flavor of BIND 9.

Here is an example scenario which fails to work as intended:

autoreconf -i
./configure --disable-maintainer-mode
make dist
tar --extract --file bind-9.17.11.tar.xz
cd bind-9.17.11
./configure --disable-maintainer-mode --enable-dnstap
make

Fix by always including pre-generated versions of all conditionally installed man pages in EXTRA_DIST. While this may cause some of them to be appear in EXTRA_DIST more than once (depending on the ./configure switches used for the build which "make dist" is run for), it appears to not be a problem for Automake.