Log Message:
Fix packages that use GITSUB_MODULES on SunOS and possibly others

This is annoying, but for now we must always explicitly combine
GITHUB_SUBMODULES with EXTRACT_USING+=bsdtar.

This is because mk/fetch/github.mk uses OPTS_TAR=--strip-components=1
and that is not supported by nbtar(pax), which is the default pkgsrc
tar on some platforms. We cannot override EXTRACT_USING in github.mk
because that is too late.

We should switch all platforms to bsdtar and retire pax.