dpkg: upgrade to 1.17.1 and fix perl scripts

 - Upgrade dpkg to 1.17.1 (most recent release, in debian unstable)

 - Disable dselect and update-alternatives, as they won't work as
   intended in homebrew.

 - Install dpkg Perl modules to $(prefix)/perl, and adjust the shebang
   on Perl scripts to point there using an existing do_perl_subst fn.