Merge manpage generation into the build system

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)

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