In 6.1, OpenBSD introduced installurl(5), which contains
the URL the sets where installed from during install or upgrade.
The content of this file is used by pkg_add(1) if PKG_PATH is
not defined.
This commit changes the behavior of __package_pkg_openbsd to omit
setting PKG_PATH to a hard-coded value if --pkg_path is not provided.
This, in turn, makes pkg_add(1) use installurl(5).
Philippe Grégoire