Add a clean pkg_add implementation on top of libarchive and libfetch.

Add a clean pkg_add implementation on top of libarchive and libfetch.
Known regressions:
- "pkg_add -" (aka reading from stdin) is currently not supported
- "pkg_add -s" is not supported either
- no progress reports for the downloads
- binary packages with hardlinks created by pkg_create before
  pkg_install-20080422 will not extract correctly (libarchive issue)
- no adhoc check for potential conflicts between dependencies and
  already installed packages

Features:
- Twice as fast for the typical case of /var/tmp and /usr/pkg on
  different filesystems
- Standalone
- implicit conflict detection before actual installation.