Allow use of native pkg_install

[jmmv]

The first version of pkg_comp 2.0 forced the use of a bootstrapped pkg_install for simplicity but this gets in the way of using binary packages on platforms where pkg_install is native (NetBSD). It also makes it impossible to mix pkg_comp-built packages with the official binary packages.

pkg_comp should be able to use the native pkg_install binaries for the final packages so that using those packages does not require having to use a different path to the package tools nor having to configure the host's pkg_path.conf file.

To consider: are there any implications on the host's pkg_install.conf file, for example, to properly configure e.g. PKG_DBDIR to match whatever was done in the pkg_comp configuration?