BUG/MINOR: config/userlist: Support one 'users' option for 'group' directive

BUG/MINOR: config/userlist: Support one 'users' option for 'group' directive

When a group is defined in a userlist section, only one 'users' option is
expected. But it was not tested. Thus it was possible to set several options
leading to a memory leak.

It is now tested, and it is not allowed to redefine the users option.

It was reported by Coverity in #2841: CID 1587771.

This patch could be backported to all stable versions.

(cherry picked from commit a1e14d2a8272511c29d9225a61c74dc89847287d)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 4c9c15e3fcf489b5fb609c6f0f1c9e06047c7cef)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>