Integrated dpkg-parsechangelog processing into Dpkg::Changelog::parse_changelog()

Integrated dpkg-parsechangelog processing into Dpkg::Changelog::parse_changelog()

* scripts/Dpkg/Changelog.pm (parse_changelog): Rewrite it completely
to not call dpkg-parsechangelog but do the work of this program by itself.
* scripts/dpkg-parsechangelog.pl: Rewrote it to use the enhanced
parse_changelog() function.
* scripts/dpkg-genchanges.pl, script/dpkg-gencontrol.pl: Adapted to use
the modified parse_changelog().
* scripts/dpkg-gensymbols.pl, scripts/dpkg-source.pl: Likewise.