net/ucx: Fix mm signal socket binding on FreeBSD

Replace Linux-style UNIX domain socket autobind logic in the mm
signaling path with explicit pathname-based binding on FreeBSD and
remove the socket path during cleanup.

This fixes FreeBSD runtime failures in the mm shared-memory signaling
path.

PR: 293867
Sponsored by: UNIS Labs
MFH: 2026Q2

(cherry picked from commit db6d744b60348636c7a87c3c7fc181afd9b71994)