Windows DNS SD : add port knocking like other implementations
Windows DNS SD : add port knocking like other implementations

Both the Linux and Mac implemations of DNS SD ensures something is there
before passing the host information up the stack. The Windows
implemnation was missing this, so add it. Add locking since it's needed
by those functions.

Fix #559

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio