BUG/MINOR: server: inherit from netns in srv_settings_cpy()

When support for 'namespace' keyword was added for the 'default-server'
directive in 22f41a2 ("MINOR: server: Make 'default-server' support
'namespace' keyword."), we forgot to copy the attribute from the parent
to the newly created server.

This resulted in the 'namespace' keyword being parsed without errors when
used from a 'default-server' directive, but in practise the option was
simply ignored.

There's no need to duplicate the netns struct because it is stored in
a shared list, so copying the pointer does the job.

This patch partially fixes GH #2038 and should be backported to all
stable versions.

(cherry picked from commit 19b5a7c7a5b4b01970e06d20928ed1d87ca6efcd)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
diff --git a/src/server.c b/src/server.c
index 1eb13c4..fc6df63 100644
--- a/src/server.c
+++ b/src/server.c
@@ -2404,6 +2404,7 @@
 	if (srv_tmpl)
 		srv->srvrq = src->srvrq;
 
+	srv->netns                    = src->netns;
 	srv->check.via_socks4         = src->check.via_socks4;
 	srv->socks4_addr              = src->socks4_addr;
 }