Log Message:
Change the way that legacy USE_* and FOO_USE_* options are converted
into the bsd.options.mk framework.  Instead of appending to
${PKG_OPTIONS_VAR}, it appends to PKG_DEFAULT_OPTIONS.  This causes
the default options to be the union of PKG_DEFAULT_OPTIONS and any
old USE_* and FOO_USE_* settings.

This fixes PR pkg/26590.