Change the linprocfs <pid>/maps and procfs <pid>/map handlers to use
sbuf instead of doing uiomove. This allows for reads from non-zero
offsets to work.

Patch is forward-ported des@' one, and was adopted to current code
by dchagin@ and me.

Reviewed by: des (linprocfs part)
PR: kern/101453
MFC after: 1 week