routine. Works similarly fto pmap_prefer(), but allows callers
to specify a minimum power-of-two alignment of the region.
How we ever got along without this for so long is beyond me.
* Remove the casts to vaddr_t from the round_page() and trunc_page() macros to
make them type-generic, which is necessary i.e. to operate on file offsets
without truncating them.
* In due course, cast pointer arguments to these macros to an appropriate
integral type (paddr_t, vaddr_t).
Originally done by Chuck Silvers, updated by myself.
thorpej