BUG/MEDIUM: master/cli: Pin the master CLI on the first thread of the group 1

BUG/MEDIUM: master/cli: Pin the master CLI on the first thread of the group 1

There is no reason to start the master CLI on several threads and on several
groups. And in fact, it must not be done otherwise the same FD is inserted
several times in the fdtab, leading to a crash during startup because of a
BUG_ON(). It happens when several groups are configured.

To fix the bug the master CLI is now pinned on the first thread of the first
group.

This patch should fix the issue #2259 and must be backported to 2.8.