Fix some issues reported by lintian

Fix some issues reported by lintian

• Set upstream metadata fields: Bug-Database, Bug-Submit. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)
• Set upstream metadata fields: Bug-Database, Bug-Submit, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)
• Update standards version to 4.6.2, 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 (out-of-date-standards-version)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/47/250e9fd67b7586e4b98f526c22d846da5152fa.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/76/ec121c2b68915f08f2abe96c461f2d95e59134.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/7c/96a3b05485e93084aefd6527a279145d7b125a.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/d3/6f61b60d9a7a73ad29190846cad00d7db2ca69.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/12/ce6a7bae65ccba2e1f39ae559eb0b203d11b5f.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/23/ce2ab58f181a4d3a526851fe278ea57b74a192.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/93/bf9ded647028e525626565725422116d6f2e1a.debug
-rw-r--r--  root/root   /usr/lib/debug/.build-id/9b/c0b408df5f06dc3cdfb2516df5e8b867f106c2.debug

No differences were encountered between the control files of package **pngtools**

Control files of package pngtools-dbgsym: lines which differ (wdiff format)

• Build-Ids: 12ce6a7bae65ccba2e1f39ae559eb0b203d11b5f 23ce2ab58f181a4d3a526851fe278ea57b74a192 93bf9ded647028e525626565725422116d6f2e1a 9bc0b408df5f06dc3cdfb2516df5e8b867f106c2 47250e9fd67b7586e4b98f526c22d846da5152fa 76ec121c2b68915f08f2abe96c461f2d95e59134 7c96a3b05485e93084aefd6527a279145d7b125a d36f61b60d9a7a73ad29190846cad00d7db2ca69

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Build and test logs for this branch can be found at https://janitor.debian.net/run/06fceb81-c0dc-4853-8d81-376de7ea467b.