unix: factor `unp_sun_path()` out of bind and connect

[inquire_JohnEricson.me]

Summary

Extract the AF_UNIX validation plus sun_path/length lookup shared by
uipc_bindat(), unp_connect(), and unp_connectat() into a helper that
hands back the path pointer and its length. Each caller keeps its own
empty-path policy and, where needed, its own copy of the path.

Signed-off-by: John Ericson <[e-mail]>
Assisted-by: Claude Code (Claude Opus 4.8 and Fable 5)

[markj]

sys/kern/uipc_usrreq.c

610

FreeBSD style puts parents around all return values.

[inquire_JohnEricson.me]

Fix the return statement to match the style guide

[inquire_JohnEricson.me]

OK I update the diff via the web form, which I noticed messes up the diff-to-diff comparison, since the base commit is gone, but hopefully that does not matter when landing this.