MINOR: server: add ws keyword

Implement parsing for the server keyword 'ws'. This is used to configure
the mode of selection for websocket protocol. The configuration
documentation has been updated.

A new regtest has been created to test the proper behavior of the
keyword.

(cherry picked from commit f9d5957cd9a94c517a9d1fea8f536ab08922d9a9)
[ad: no support for dynamic servers for error-limit keyword]
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
diff --git a/doc/management.txt b/doc/management.txt
index 38af652..eaa4835 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1476,6 +1476,7 @@
   - tfo
   - usesrc
   - weight
+  - ws
 
   Their syntax is similar to the server line from the configuration file,
   please refer to their individual documentation for details.