Always build with DEBUG and enable assertions.
xbps_repository_is_remote: optimize and check for http:// first.
Optimize repository API functions.

- Remove xbps_repo_get_plist() and try to internalize all members at
  xbps_repo_open() time.

- Added xbps_repo_open_idxfiles() to also internalize the index-files
  plist from repository, which is really huge and must only be internalized
  when needed.

- Improve how signed and verified repositories are detected.

- Misc optimizations and small performance improvements.

Bump XBPS_API_VERSION.
xbps-query: adapt to the new libxbps API.
xbps-rindex: remove -c --clean mode and adapt to the new libxbps API.

- Removed -c --clean mode. It's not too useful and adds a considerable
  amount of gratuitous code. It takes almost the same time than
  adding all pkgs from scratch.

- When creating the repository data always add the meta plist at the
  2nd position in the archive, to optimize its access.

- Misc improvements.