Add capability package

[kolyshkin]

This integrates github.com/kolyshkin/capability (which itself is a fork
of github.com/syndtr/gocapability) to github.com/moby/sys/capability,
as suggested in opencontainers/runc#4358 (comment)

Some of github.com/syndtr/gocapability users are (in an alphabetical order):

• https://github.com/canonical/lxd
• https://github.com/containers/buildah
• https://github.com/containers/podman
• https://github.com/containers/skopeo
• https://github.com/google/gvisor
• https://github.com/hashicorp/nomad
• https://github.com/linuxkit/linuxkit
• https://github.com/opencontainers/runc
• https://github.com/opencontainers/runtime-tools
• https://github.com/slimtoolkit/slim

The way it was done is:

# 1. Create a copy of a local repo: 
git clone --no-local capability capability-for-merge
 
# 2. Use git-filter-branch to:
#  - move contents to a subdirectory
#  - remove useless merge commits
#  - remote .github files
#  - add 'capability: ' prefix to commit messages
git filter-repo --prune-degenerate always --path-rename :capability/ \
    --path-match .github --invert-paths \
    --message-callback '
  prefix = b"capability: "
  if not message.startswith(prefix):
    message=prefix+message
  return message
'

# 3. Merge the above repo to moby/sys
cd ~/git/moby/sys
git checkout -b add-capability
git remote add cap-merge ../../capability-for-merge
git fetch cap-merge
git merge --allow-unrelated-histories --signoff -S cap-merge/main

[kolyshkin]

I guess we're OK with the DCO check failing, it would be unwise to rewrite 12 year old commits.

[thaJeztah]

I marked the DCO to pass; for posterity; these are the commits that failed the DCO;