unix: inline unp_connect_peer() into unp_connectat()

[glebius]

Summary

The unp_connect_peer() is used only once and the contract with
unp_connectat() was always hard to understand. Inlining, we got isolation
of UNP_CONNECTING flag and also more clear understanding when peer socket
reference needs to be released, and when not. Also, reading linear
sequence of the operation is easier than jumping across two functions.