DOC: Clarify documentation of option lb-agent-chk

Avoid referring to check-port as this is not a configuration parameter.

Signed-off-by: Simon Horman <horms@verge.net.au>
CLEANUP: Make parameters of srv_downtime and srv_getinter const

The parameters of srv_downtime and srv_getinter are not modified
and thus may be const.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Split up struct server's check element

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.

The split has been made by:
* Moving elements of struct server's check element that will
  be shared by both checks into a new check_common element
  of struct server.
* Moving the remaining elements to a new struct check and
  making struct server's check element a struct check.
* Adding a server element to struct check, a back-pointer
  to the server element it is a member of.
  - At this time the server could be obtained using
    container_of, however, this will not be so easy
    once a second struct check element is added to struct server
    to accommodate an agent health check.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Move {,fast,down}inter to struct check

Move {,fast,down}inter elements from struct server to struct check.
This allows those elements of a check to be independent of the check's server.

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Move result element to struct check

Move result element from struct server to struct check
This allows check results to be independent of the check's server.

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Paramatise functions over the check of a server

Paramatise the following functions over the check of a server

* set_server_down
* set_server_up
* srv_getinter
* server_status_printf
* set_server_check_status
* set_server_disabled

Generally the server parameter of these functions has been removed.
Where it is still needed it is obtained using check->server.

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.
By paramatising these functions they may act on each of the checks
without further significant modification.

Signed-off-by: Simon Horman <horms@verge.net.au>

---

This is a rather large patch and I would be happy to split it up
on request.
MEDIUM: cfgparse: Factor out check initialisation

This is in preparation for struct server having two elements
of type struct check.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Add state to struct check

Add state to struct check. This is currently used to store one bit,
CHK_RUNNING, which is set if a check is running and clear otherwise.
This bit was previously SRV_CHK_RUNNING of the state element of struct
server.

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
MEDIUM: Add name element to struct check

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
MEDIUM: Move health element to struct check

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Add type type to struct check

This is used to indicate the type of a check independent of
its server's proxy's check type.

This is in preparation for associating a agent check
with a server which runs as well as the server's existing check.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Add helper for task creation for checks

This helper is in preparation for adding a second struct check element
to struct server.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: checks: Add supplementary agent checks

Allow an agent check to be run in conjunction with one other server
health check.

If the backend for a server check is not lb-agent-chk then an agent
check may also be run using the agent-check parameter to a server,
which sets the TCP port to be used for the agent check.

e.g.
server	web1_1 127.0.0.1:80 check agent-port 10000

The agent-inter parameter may also be used to specify the interval
and timeout for agent checks.

If either the health or agent check determines that a server is down
then it is marked as being down, otherwise it is marked as being up.

Signed-off-by: Simon Horman <horms@verge.net.au>