MFC r334656, r334665, r334695

r334656:
Add vsnprintf() to libsa. Alpha-sort the printf prototypes in stand.h.

r334665:
Make the v*printf() functions in libsa return int instead of void.
This makes them compatible with the C standard signatures, avoiding
spurious mismatch errors in the places where the oddball requirements
of standalone code end up putting two declarations of the same function
in play.

r334695:
Remove comments and assertions that are no longer valid after r330809.

r330809 replaced duplication of devdesc struct fields with an embedded copy
of the devdesc struct, to avoid fragility. That means all the scattered
comments indicating that structs must match are no longer valid. Likewise
asserts that attempted to mitigate some of the old fragility.

Reviewed by: imp@