Fix FTBFS.
    
    - Eliminate libvfork, PKGLIBDIR (Abandoned upstream).
    - Rework perl-BRs.
    - Reflect upstream installing perl-modules into perl_vendordir.
    - Reflect upstream installing bash-completion into /usr/share/bash-completion.
    - BR: /usr/bin/dpkg-buildflags, /usr/bin/dpkg-vendor, /usr/bin/dpkg-parsechangelog.
    - BR: pkgconfig(bash-completion).
    - Remove archpath, whodepends's man-pages.
    - Rebase patches.