MINOR: server: Make 'default-server' support 'check' keyword.

Before this patch 'check' setting was only supported by 'server' directives.
This patch makes also 'default-server' directives support this setting.
A new 'no-check' keyword parser has been implemented to disable this setting both
in 'default-server' and 'server' directives.
Should not break anything.
diff --git a/include/types/server.h b/include/types/server.h
index feede6d..c973d69 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -240,6 +240,7 @@
 	int puid;				/* proxy-unique server ID, used for SNMP, and "first" LB algo */
 	int tcp_ut;                             /* for TCP, user timeout */
 
+	int do_check;                           /* temporary variable used during parsing to denote if health checks must be enabled */
 	struct check check;                     /* health-check specific configuration */
 	struct check agent;                     /* agent specific configuration */