gh-154324: Fix os.sendfile() error reporting on illumos (GH-154327)

gh-154324: Fix os.sendfile() error reporting on illumos (GH-154327)

illumos sendfile(3EXT) returns the number of transferred bytes by adding
it to the offset without initializing it, so the offset cannot be used to
detect a partial write.  Use sendfilev(3EXT), which reports the number of
transferred bytes in an explicit out parameter.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>