Threads are the recommended way to scale CPU performance since HAProxy
1.8.

Official documentation says: « While "nbproc" historically used to be the only
way to use multiple processors, it also involved a number of shortcomings
related to the lack of synchronization between processes (health-checks, peers,
stick-tables, stats, ...) which do not affect threads. As such, any modern
configuration is strongly encouraged to migrate away from "nbproc" to
"nbthread". ».

While more recent versions of HAProxy automatically detect the number of
available CPU and enable threads for them, it can be useful to explicitely set
the value.

In this patch, setting cpu-map for threads is not supported.

Change-Id: Id917c70f3dbe52f24f25d9403ba8151729e8966b