new pre-dependency: perl{,-base,-modules} -> dpkg (>= 1.17.17)

[Niko Tyni <ntyni@debian.org>]

In order to fix trigger related wheezy->jessie upgrade failures in
xfonts-traditional (#774844, cc'd), I intend to make the main perl
binary packages (perl, perl-base, and perl-modules) Pre-Depend on dpkg
(>= 1.17.17), which has this change:

  * Defer trigger processing if the package does not fulfill dependencies.
    Closes: #671711

Together with making the jessie perl-modules and perl-base Break the
wheezy perl, this should ensure that the xfonts-traditional trigger will
not run when perl is in a broken state during upgrades.

Please see the #774844 bug log for details, and let me know if you have
objections or other suggestions.
-- 
Niko Tyni   ntyni@debian.org