MINOR: dns: Handle SRV records.

Make it so for each server, instead of specifying a hostname, one can use
a SRV label.
When doing so, haproxy will first resolve the SRV label, then use the
resulting hostnames, as well as port and weight (priority is ignored right
now), to each server using the SRV label.
It is resolved periodically, and any server disappearing from the SRV records
will be removed, and any server appearing will be added, assuming there're
free servers in haproxy.
diff --git a/include/types/server.h b/include/types/server.h
index 724d496..77263db 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -298,6 +298,7 @@
 		int nb_low;
 		int nb_high;
 	} tmpl_info;
+	struct dns_srvrq *srvrq;		/* Pointer representing the DNS SRV requeest, if any */
 };
 
 /* Descriptor for a "server" keyword. The ->parse() function returns 0 in case of