This ensures the installed manpages always match the installed version,
and simplifies the release process. An optional dependency on `help2man`
is added (default on, can be avoided with `-DENABLE_MAN=OFF`).

Since the autotools build system is deprecated, manpages are simply
removed from the autotools build.

Manpages are intended to be used on Unix-like platforms. Therefore, the
build instructions for Windows include `-DENABLE_MAN=OFF`, and in Gitian
manpages are now excluded from the Windows releases (zip file and
installer).

This also fixes a bug in the Gitian build where it would unnecessarily
extract the source tarball without using the extracted stuff.

Test plan:

* `./autogen.sh;./configure --enable-deprecated-build-system;make
install`
* `cmake -GNinja . -DENABLE_MAN=OFF;ninja install`
* `cmake -GNinja . -DENABLE_MAN=ON;ninja install;man bitcoind`
* Gitian builds for all platforms (verify hashes, inspect
Linux/MacOS/Windows zip files, try Windows installer)