dns: warn that the BIND plugin is experimental

It's still got a bunch of issues that need debugging, like when VPN
nameservers exist but no domain and thus not doing split DNS, sometimes
hosts outside the VPN don't resolve correctly, which was previously
masked by having the non-VPN nameservers in /etc/resolv.conf where
glibc would erroneously use them instead of asking BIND.  To be fixed
in a subsequent patch.

The dnsmasq plugin seems to work great though.

man/NetworkManager.conf.5.in

@@ -117,10 +117,6 @@ DNS data to applications that use it.
 .TP
 .I dnsmasq
 this plugin uses dnsmasq to provide local caching nameserver functionality.
-.TP
-.I bind
-this plugin uses the ISC BIND program to provide local caching nameserver
-functionality.
 .RE
 .SS [keyfile]
 This section contains keyfile-specific options and thus only has effect when using \fIkeyfile\fP plugin.

src/dns-manager/nm-dns-manager.c

@@ -1007,9 +1007,10 @@ load_plugins (NMDnsManager *self, const char **plugins)
 		for (iter = plugins; iter && *iter; iter++) {
 			if (!strcasecmp (*iter, "dnsmasq"))
 				plugin = NM_DNS_PLUGIN (nm_dns_dnsmasq_new ());
-			else if (!strcasecmp (*iter, "bind"))
+			else if (!strcasecmp (*iter, "bind")) {
 				plugin = NM_DNS_PLUGIN (nm_dns_bind_new ());
-			else {
+				nm_log_warn (LOGD_DNS, "The BIND plugin is experimental!");
+			} else {
 				nm_log_warn (LOGD_DNS, "Unknown DNS plugin '%s'", *iter);\
 				continue;
 			}