Strengthen the sanity checking of busdma tag parameters.

It turns out an alignment of zero can lead to an endless loop in the
vm reservations code, so specifically disallow that.  The manpage says
hardware which can do dma at any address should use a value of one, which
hints at the forbiddeness of zero without exactly saying it.  Several
other conditions which could lead to insanity in working with the tag are
also checked now.

Every existing call to bus_dma_tag_create() (about 680 of them) was
eyeballed for violations of these things, and two alignment=0 glitches
were fixed.  It's possible something was missed, but overall this
shouldn't lead to any arm users suddenly experiencing failures.