tun.c: enclose DNS domain in single quotes in WMIC call

This is needed to support domains with hyphens.

Not using double quotes here, since our code replaces
them with underbars (see
https://github.com/OpenVPN/openvpn/blob/master/src/openvpn/win32.c#L980).

Fixes OpenVPN#363

Change-Id: Iab536922d0731635cef529b5caf542f637b8d491
Signed-off-by: Lev Stipakov <lev@openvpn.net>

src/openvpn/tun.c

@@ -333,7 +333,7 @@ do_dns_domain_wmic(bool add, const struct tuntap *tt)
     }
 
     struct argv argv = argv_new();
-    argv_printf(&argv, "%s%s nicconfig where (InterfaceIndex=%ld) call SetDNSDomain %s",
+    argv_printf(&argv, "%s%s nicconfig where (InterfaceIndex=%ld) call SetDNSDomain '%s'",
                 get_win_sys_path(), WMIC_PATH_SUFFIX, tt->adapter_index, add ? tt->options.domain : "");
     exec_command("WMIC", &argv, 1, M_WARN);