MINOR: tcp/http/conf: extends the keyword registration options

This patch permits to register a new keyword with the keyword "tcp-request content"
'tcp-request connection", tcp-response content", http-request" and "http-response"
which is identified only by matching the start of the keyword.

for example, we register the keyword "set-var" with the option "match_pfx"
and the configuration keyword "set-var(var_name)" matchs this entry.
diff --git a/include/types/proto_tcp.h b/include/types/proto_tcp.h
index a6af2d3..d34ff03 100644
--- a/include/types/proto_tcp.h
+++ b/include/types/proto_tcp.h
@@ -65,6 +65,7 @@
 	const char *kw;
 	int (*parse)(const char **args, int *cur_arg, struct proxy *px,
 	             struct tcp_rule *rule, char **err);
+	int match_pfx;
 };
 
 struct tcp_action_kw_list {