libbsnmp: fix local client socket creation

Revert "bsnmp: Use mkstemp when creating clients local socket"

The use of mktemp() was intentional here. We don't want a regular file to
be created. If created, it guarantees that following bind(2) would always
fail with EADDRINUSE and thus open_client_local() always fails.

This reverts commit 81e0e7b9e36d6a25b3af6482811318e085537d2f.

Reviewed by: rew, harti
Differential Revision: https://reviews.freebsd.org/D51031