(#16752) Add support for pkg.conf on openbsd

Previously only local file sources were supported for openbsd
packages. Pkg.conf is a mechanism that openbsd uses to download
packages from HTTP sources as well. This patch adds checking for
the presence of 'installpath' lines in pkg.conf and, if one
are found and if the last character of an installpath line is a
::File::SEPARATOR (aka `/` character on openbsd) then it will
become the value of the environment variable PKG_PATH previous to
pkgadd invocation, which tells pkgadd to use the URL.