Both doc/man/ddns-confgen.rst and doc/man/tsig-keygen.rst include
bin/confgen/tsig-keygen.rst, which defines a "man_tsig-keygen" label.
This triggers the following warning when running sphinx-build with the
-W command line switch in the doc/man/ directory:

    ../../bin/confgen/tsig-keygen.rst:27: WARNING: duplicate label man_tsig-keygen, other instance in /tmp/bind9/doc/man/ddns-confgen.rst

Move the offending label from bin/confgen/tsig-keygen.rst to the proper
spot in doc/arm/manpages.rst to avoid effectively defining it twice in
different source documents while still allowing the relevant man page to
be referenced in the ARM.  Also rename that label so that it more
closely matches the content it points to.  As the label no longer
immediately precedes a section title in its new location, use
:ref:`Title <label>` syntax for the only reference to the
tsig-keygen/ddns-confgen man page in the ARM.