Some packaging improvements

I needed an upstream patch to test some upstreaming work and took that opportunity to make some improvements to the packaging.
I've created them as individual commits, but feel free to (ask me to) squash them (or make adjustments).

• control: Remove unused Build-Depends
• control: Update Standards-Version to 4.6.2
• control: Specify Rules-Requires-Root: no
• control, rules: Drop quilt B-D and param
• control: Update Homepage to point to gitlab.f.o
• patches: Add DEP-3 header to 01_default_perms.diff
• symbols: Add missing Build-Depends-Package fields
• symbols, lintian: Drop reference to private symbols

I also noticed the following things:

1. (Project) Settings -> CI/CD -> General pipelines refers to a non-existing configuration file. If you change that to recipes/debian.yml@salsa-ci-team/pipeline then you'll notice it already successfully passes the CI pipeline
2. If you add tags in the form of upstream/<version> to the upstream-unstable branch, then you can do gbp export-orig to generate the orig tarball (and don't have to use apt-get source libdrm to get it, which then complains that one should use the git repo)
3. The meson.build file checks for atomic_ops as optional dependency. If one installs libatomic-ops-dev (and/or add it to B-D), then that dependency gets fulfilled and it builds successfully. As I don't know if this is desired (or good), I didn't include it in this MR's commits
4. It also checks for the optional cunit dependency, but when I installed libcunit1-dev the build actually failed. Due to that and similar reasoning as previous item, I didn't include that part in this MR either.