exterror(9): add infra for bufs and bios

MFC note: this commit changes the layout for both struct buf and bio.
As such, it breaks KBI for VFS, which by itself often happens after X.0,
so this is not a precedent.

Discussed with: mckusick
Approved by: re (cperciva)

(cherry picked from commit 6c406b5b93125d030f0e63716ff389ce1a6ec4c5)