rpc.tlsservd.c: Pin max threads at 1 for now

PR#289734 reports a crash in tcp_use_close() when the rpc.tlsservd
daemon is being used. Although I have no, as yet, had time to track
down/fix the race, it turns out setting rpctls_maxthreads to 1
avoids the problem.

This patch pins rpctls_maxthreads to 1 to avoid the problem,
until there is a proper fix.

PR: 289734
MFC after: 3 days