INTR_SOLO was introduced in 2016, but the author apparently wasn't quite
ready to push it. This looks like a good way forward, so test some and
find issues which have cropped up over time.
Some ARM functions can simply be cast. They're using softc which uses
a compatible prototype. The function intr_irq_handler() lost its
support (or never had it).
Start adding DISABLE_TD_INTR_FRAME since removing td_intr_frame is the
way towards the future.