unix/stream: provide own pr_sopoll

[glebius]

Summary

This version would check into the peers buffer if connected. It uses
protocol lock to check for SOLISTENING(so), which is safe since conversion
uses both locks.

Fixes tests/kern/unix_stream:full_not_writable

This shall be squashed into the main new unix(4) commit.

[glebius]

squashed into D48921