Fix some issues reported by lintian

Fix some issues reported by lintian

• Remove 2 obsolete maintscript entries in 1 files.
• Bump debhelper from old 12 to 13. (package-uses-old-debhelper-compat-version)
• Set upstream metadata fields: Bug-Database, Bug-Submit, Contact, Repository, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking, upstream-metadata-missing-repository)
• Update standards version to 4.6.0, no changes needed. (out-of-date-standards-version)
• Avoid explicitly specifying -Wl,--as-needed linker flag. (debian-rules-uses-as-needed-linker-flag)
• Remove 2 obsolete maintscript entries in 1 files.
• Set upstream metadata fields: Bug-Database, Bug-Submit, Contact, 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 (out-of-date-standards-version)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/d4/5f5c5a9e8c0026fa61a64da2574b13af76d06c.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/14/14c3dd4324cfa1f10eb9ea68ed2666f540944c.debug
-rwxr-xr-x  root/root   DEBIAN/postinst
-rwxr-xr-x  root/root   DEBIAN/postrm
-rwxr-xr-x  root/root   DEBIAN/preinst
-rwxr-xr-x  root/root   DEBIAN/prerm

No differences were encountered between the control files of package **gnome-power-manager**

Control files of package gnome-power-manager-dbgsym: lines which differ (wdiff format)

• Build-Ids: 1414c3dd4324cfa1f10eb9ea68ed2666f540944c d45f5c5a9e8c0026fa61a64da2574b13af76d06c

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Build and test logs for this branch can be found at https://janitor.debian.net/run/56a67f39-52cb-43c8-a94a-07e1aaef004d.