unix/stream: repair SO_SNDTIMEO

The send operations are waiting on the peer's socket buffer, but we shall
use our timeout value. Provide a test for that.

Reported by: phk
Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D53081
Fixes: d15792780760ef94647af9b377b5f0a80e1826bc

(cherry picked from commit ead721935251ab5640ea736d4269814b7f9d2f64)