BUG/MINOR: resolvers: Unlink DNS resolution to set RMAINT on SRV resolution

BUG/MINOR: resolvers: Unlink DNS resolution to set RMAINT on SRV resolution

When a server is set in RMAINT becaues of a SRV resolution failure, the
server DNS resolution, if any, must be unlink first. It is mandatory to
handle the change in the context of a SRV resolution.

This patch must be backported as far as 2.2.

(cherry picked from commit bca680ba90787fc57b9711e268f6fb0c74e7e49c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 833937da99caf3639e67a14409095a402ea2f0b2)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit f586f823539583c5f637e239a90f6dc0a959b859)
[cf: Must be backported as far as 2.0 because of recent changes on resolvers]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>