BUG/MINOR: server: Properly handle init-state value during haproxy startup

BUG/MINOR: server: Properly handle init-state value during haproxy startup

Unlike stated in the configuration manual, the server 'init-state' parameter
was not evaluated during haproxy startup/reload. After a review, it appeared
there were also issues if combined with the 'track' parameter. In addtition,
this parameter was only evaluated when health-checks were enabled for the
server, leading to unexpected behavior if the serve settings are dynamically
changed via the CLI.

To fix those issues, behavior of the 'init-state' parameter was slightly
adapted. It is always evaluated, even when there is no running health-checks
for the server. An error is reported if the 'track' parameter is also
defined. Both cannot work together.

In addition, the "none" state was introduced to be able to restore the
default behavior. It will be especially useful when the parameter is
inherited from a 'default-server' directive.

This patch should fix the issue #3298. It must be backported as far as 3.2.