Fix some issues reported by lintian

Fix some issues reported by lintian

• Set upstream metadata fields: Bug-Database, Bug-Submit, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)
• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)
• Set upstream metadata fields: Repository. (upstream-metadata-missing-repository)
• Update standards version to 4.6.2, no changes needed. (out-of-date-standards-version)

Debdiff

These changes affect the binary packages:

[The following lists of changes regard files as different if they have different names, permissions or owners.]

Files in second set of .debs but not in first

-rw-r--r--  root/root   /usr/lib/debug/.build-id/97/5dcdc38156c5040946e604fd392701bb204287.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/7d/3bc142622e31339a0e551fdad9c05c66592d3b.debug

No differences were encountered between the control files of package **libzn-poly-0.9**

Control files of package libzn-poly-0.9-dbgsym: lines which differ (wdiff format)

• Build-Ids: 7d3bc142622e31339a0e551fdad9c05c66592d3b 975dcdc38156c5040946e604fd392701bb204287

No differences were encountered between the control files of package **libzn-poly-dev**

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/libzn-poly/39522f12-14d7-4317-b99d-0edb0931e872.