use local changelog from /usr/share/doc if possible

use local changelog from /usr/share/doc if possible

If pkgAcqChangelog is told to acquire the changelog for a version it
will check first if this version is installed on the disk and if so will
use the local changelog in /usr/share/doc (possibily/likely gz
compressed) instead of downloading the file from the web.

An option is provided to disable this, which is enabled by default for
the Ubuntu vendor as they truncate the local changelogs – and for apts
--print-uris action.