v1.4.0

Release v1.4.0

- drop the libcap dependency: only the CAP_* constants were used and they also
  come from <linux/capability.h>, so there is no external dependency anymore
  (the bundled libcap-ng is all that is needed)
- fix a crash in set_user(): caps_to_drop.back() was read on an empty vector
  when the first capability was permitted-but-not-effective
- fix validate_ambient(): it self-assigned (this->ambient = ambient) instead of
  storing the cleaned set, so it never actually pruned the ambient set
- fix the dead `gid >= 0` check (gid_t is unsigned, so it was always true); a
  gid of (gid_t)-1 now correctly skips the group change
- remove the leftover include/capabilities.hpp.orig
- update GitHub Actions (actions/checkout@v5, create/upload-asset actions @v2;
  drop the now-unneeded libcap-dev install and the empty submodule step)
- expand the README with a usage example

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>