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

: Update =>

Changelog:

PR:
Sponsored by: UNIS Labs